Pneumatic transport and heat exchange systems

ABSTRACT

Gas-solids transport and heat exchange techniques are disclosed wherein solid particulate material is circulated in a &#34;figure 8&#34; or a circular flow path for selective contact and/or direct heat exchange with gaseous media. The particulate material is introduced into streams of gaseous media at spaced locations in the flow path and subsequently separated from the gaseous streams following contact and/or heat exchange therewith. The gaseous streams are maintained separate from one another by loose packed bed columns of particulate material formed in the flow path and used to introduce the particulate material into the gaseous streams. The flow rate of the particulate material is regulated by the controlled biasing of particulate material from each of the columns thereof directly into the gaseous streams, and the particulate material is circulated solely through the use of the gaseous media and the force of gravity. The particulate material is circulated in cocurrent relationship with each of the gaseous streams in figure 8 flow path systems and, in circular flow path systems, the particulate material is circulated in cocurrent relationship with one of the gaseous streams and in countercurrent relationship with the other of the gaseous streams. In heat exchange applications, heat transfer between the streams of gaseous media is provided as a function of the flow rate of the particulate material and the relative flow rates of the streams of gaseous media.

This application is a continuation-in-part of Ser. No. 942,677, filedSept. 15, 1978, now abandoned.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention generally relates to gas-solids transporttechniques and heat exchange techniques wherein a solid particulatematerial suitably sized for pneumatic conveyance is transported orcirculated for selective contact and/or direct heat exchange with fluidor gaseous media. Loose packed bed flow and pneumatic conveyance,together with the force of gravity, are employed to transport theparticulate material. The transport and heat exchange techniques areparticularly useful for recovering heat from high temperature processgases such as flue gas and, in preferred heat exchange systems, theparticulate material is circulated in a "figure 8" or a circular "zeroloop" flow path or pattern and heat transfer between separate streams ofgaseous media is provided as the particulate material contacts onegaseous stream and then the other. The gaseous streams are maintainedseparate by means of loose packed bed columns of particulate materialformed in the flow path and used to introduce the particulate materialinto the gaseous streams.

The catalytic cracking of hydrocarbons has resulted in a number of priorart gas-solids transport techniques wherein the solid comprises aparticulate catalyst which is circulated between a reactor cycle and aregenerator cycle in a figure 8 flow pattern. In addition to hydrocarboncracking, similar gas-solids transport systems are employed for otherchemical processes. The control of particulate material flow in suchprior art systems generally include regulation of the gravity withdrawalof the particulate material through standpipes or downcomers with theuse of mechanical valves. The prior art also discloses the monitoring ofpressure differentials across light and dense phases of fluidized bedsfor feed purposes and across valves for purposes of regulating the flowof particulate material therethrough.

The subject gas-solids transport techniques are well illustrated in thedisclosed heat exchange systems, methods and apparatuses hereinafterdescribed in detail. The heat exchange systems have been foundespecially advantageous in severe temperature and environmentapplications such as the recovery of heat from flue gases attemperatures substantially higher than 1000° F. In such severeapplications, the prior art heat recovery systems are substantiallylimited to heat wheels, regenerative devices characterized by brickarrangements, and pebble devices. These prior art devices are not ofparticular concern herein, and they essentially represent differentclasses of heat exchangers and heat exchanger techniques characterizedby substantial capital investments and space requirements.

SUMMARY OF THE INVENTION

As previously indicated, the present invention contemplates gas-solidstransport and heat exchange techniques wherein the solid particulatematerial is transported by means of pneumatic conveyance and loosepacked bed flow. The particulate material is conveyed throughsubstantially unobstructed passageways or conduits, and the flow rate ofthe particulate material is regulated to obtain optimized transportand/or heat exchange conditions during pneumatic conveyance and loosepacked bed flow. The transport and heat exchange systems embodying thedisclosed techniques readily lend themselves to automatic control andenable preselection of system operating characteristics.

In accordance with the subject invention, the particulate material iscirculated in a continuous flow path or pattern and flows or streams ofgaseous media are established at spaced locations in the flow path fordirect contact and/-or heat exchange with the particulate material.Loose packed bed columns of particulate material are formed in the flowpath for providing dynamic seals between the streams of gaseous mediaand for introducing the particulate material into the gaseous streams.The particulate material is separated from each gaseous stream followingcontact therewith and is substantially continuously circulated along theflow path including the gaseous stream contacting portions thereof. Theillustrated systems include cocurrent and countercurrent contact withgaseous media.

In the illustrated embodiments wherein the particulate material iscirculated in a "figure 8" flow pattern between two vessels, a downcomeris used to withdraw particulate material from an inventory thereofcontained within each of the vessels. The particulate material iswithdrawn under the influence of gravity and a flowing loose packed bedor bed column of particulate material is formed in a substantiallyvertical portion of each of the downcomers. Each of the downcomers alsoincludes a laterally extending portion for accumulating the particulatematerial adjacent the bottom of each of the columns and transferring theparticulate material to associated lift lines. The particulate materialis pneumatically conveyed to one of the vessels by a first gaseousstream flowing through one of the lift lines and to the other of thevessels by a second gaseous stream flowing through the other of the liftlines. The particulate material is separated from the gaseous streamsfollowing pneumatic conveyance and respectively collected in theinventories of particulate material contained within the vessels.

In the illustrated embodiment wherein the particulate material iscirculated in a circular or "zero loop" flow path between two vessels,downcomers of a slightly modified structure and loose packed bed columnsare again used to withdraw particulate material from flowing inventoriesthereof maintained in the vessels. In this instance, one of thedowncomers is arranged to deliver particulate material into an upperregion of one of the vessels for downward flow thereof due to the forceof gravity and countercurrent contact with a first gaseous streamflowing upwardly through the vessel. Following contact with the firstgaseous stream, the particulate material is withdrawn from a lowerregion of the vessel through the other downcomer and transferred to alift line for cocurrent contact with a second gaseous stream as it ispneumatically conveyed to the upper region of the other one of thevessels.

The bulk density of the flowing column or bed of particulate material iscontrolled or maintained to prevent the net leakage of gaseous mediathrough the downcomers as well as the flow of particulate material duesolely to the force of gravity and any difference of the gas pressuresat opposite ends of the downcomer. The downcomers are arranged so thatthe particulate material may be biased to directly spill into theassociated lift lines or gaseous streams by a minimal disturbance of theparticulate material from its angle of repose adjacent the bottom ofeach of the downcomers.

In the pneumatic conveyance of the particulate material, the pressuredrop of the gaseous conveying medium has been found to be sufficientlyclosely related to the relative amount of particulate material beingconveyed to warrant the use of pressure drop as a control parameter forregulating the flow of particulate material. Accordingly, preferredsystems include regulation of the withdrawal of particulate materialthrough at least one downcomer as a function of the pressure drop acrossits associated lift line. In addition to pressure drop, other sensedoperating variables such as outlet temperatures may be used for controlor combined with pressure drop regulation to provide a combinedparameter control system.

In heat exchange applications wherein heat is to be transferred from aprimary fluid or heat providing gaseous stream to a secondary fluid orheat receiving gaseous stream, the primary and secondary streams areeach used to pneumatically convey the particulate material through oneof the lift lines to its associated operating vessel in figure 8systems. In zero loop systems, the particulate material flows downwardlythrough the primary gaseous stream while undergoing countercurrent heattransfer therewith and the secondary gaseous stream is used topneumatically convey the particulate material through the lift linewhile undergoing cocurrent heat transfer therewith. In each system,cocurrent heat transfer between the particulate material and the gaseousstream is substantially completed during its pneumatic conveyance by thegaseous stream.

The particulate material provides a heat exchange surface of variablearea which can be varied internally with respect to the heat exchangersystem by modulation of the flow rate of the particulate material or theweight ratio of solids to gaseous medium. Upon modulation of the flowrate, corresponding changes occur in the inventories of particulatematerial in the vessels since the total amount of particulate materialis constant. Accordingly, the particulate material flow rate may bemodulated to vary the quantity of heat transferred and the outlettemperature of the secondary fluid. If the pressure drop of the systemto which the heat exchanger is applied is a limiting parameter, the heatexchanger may be operated at a constant total system pressure drop withautomatic variation of the flow rate of particulate material. The weightratio of the flow rate of the secondary fluid to the primary fluid canalso be varied to provide a desired outlet temperature or quantity ofheat transfer.

It has also been discovered in heat exchange applications that the rateof heat transfer can be maximized as a direct function of the slipvelocity or velocity difference between the gaseous medium andparticulate material during pneumatic conveyance. Accordingly, theparticulate material is caused to violently mix with the gaseous streamduring pneumatic conveyance to maximize the slip velocity in preferredheat exchange systems. The violent mixing can be localized at one ormore locations as the particulate material is being pneumaticallyconveyed with corresponding increases in the rate of heat transfer.

The maximization of the rate of heat transfer by means of slip velocitymaximization enables the heat exchanger to be optimally sized for apreselected level of heat exchange with minimization of the heatexchange surface area or solids flow rate and system pressure drop.Further, the gas-solids transport techniques are employed to maximizethe efficiency of the exchanger system as a whole by minimizing systempressure drops other than those desired during pneumatic conveyance toenhance the velocity difference between the particulate material and thegaseous stream.

In contrast with the prior art, the subject gassolids transport systemsenable controllable steady state operation in a functionally andstructurally efficient manner. The gravity withdrawal of particulatematerial through the downcomer in a loose packed bed of controlleddensity provides an effective dynamic seal against gas pressure biasedbulk leakage of the gaseous media which accommodates variations in theselected operating parameters of the system as well as externallyimposed operating fluctuations. The accumulations of particulatematerial in the lateral transfer portions of the downcomers functionallyprovide a gravity feed-lock system which enables the flow of theparticulate material to be controlled without the necessity ofconventional valves or similar mechanical flow controllers and actuatorstherefor. The elimination of valves and other such mechanical devices isalso advantageous since it avoids the maintenance problems of suchdevices as well as the increased pressure drops associated therewith andcorrespondingly increased power requirements for the system.

In heat exchange applications, the subject systems represent a new classof heat exchangers as compared with presently available commercialdevices. In contrast with the relatively large and expensive prior artdevices, a 500,000 Btr/hr. heat exchanger figure 8 flow path unitcapable of direct recovery of heat from flue gas at temperatures inexcess of 1000° F. in accordance with the present invention can beprovided with minimum dimensions of about 2 ft.×3 ft.×10 ft. and at afraction of the cost of prior art devices of comparable capacity andcapability. Further, the capacity of the heat exchanger unit may besignificantly increased with only modest increases in cost and size.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view, partially in section, showing a heatexchanger system having a figure 8 flow path in accordance with thepresent invention;

FIG. 2 is a sectional view taken along the line 2--2 in FIG. 1, withparts omitted for clarity of illustration.

FIG. 3 is an elevational view, partially in section and on an enlargedscale, showing the lower portion of a downcomer and its associated liftline;

FIG. 4 is an elevational view similar to FIG. 3 showing a modifieddowncomer arrangement;

FIG. 5 is a sectional view plan view on an enlarged scale showing thetangential feed of the solid particulate material from a downcomer to anassociated lift line;

FIG. 6 is an elevational view, partially in section, showing a lift lineand its associated bonnet;

FIG. 7 is a schematic view of a downcomer and associated lift line and acontrol arrangement for regulating the withdrawal of particulatematerial through the downcomer as a function of pressure drop across thelift line and/or outlet temperature;

FIG. 8 is a schematic view of a lower portion of an operating vessel, adowncomer for withdrawing particulate material thereform and itsassociated lift line, and a control arrangement for regulating thewithdrawal of particulate material as a function of the inventory ofparticulate material above the downcomer;

FIG. 9 is a diagrammatic view of the apparatus shown in FIG. 1illustrating the location of sensed temperature and pressure operatingvariables;

FIG. 10 is a graph illustrating the relationship between pressure dropand the flow of particulate material;

FIG. 11 is a graph illustrating the rates of change or sensitivities ofthe air and flue gas outlet temperatures with respect to the flow rateof particulate material;

FIG. 12 is a graph illustrating the percent heat recovery as a functionof the total pressure drop across the lift lines of the heat exchanger;

FIG. 13 is a graph illustrating the theoretical responsiveness of theair outlet temperature to equal fractional changes in the air andparticulate material flow rates at various operating conditions for theheat exchanger;

FIG. 14 is a graph illustrating the theoretical percent heat recovery asa function of the air, flue gas and particulate material flow rates;

FIG. 15 is a graph illustrating the percent heat recovery achieved bythe heat exchanger as a function of the total pressure drop across thelift lines;

FIG. 16 is a graph illustrating the temperature profiles of the air andflue gas during pneumatic conveyance;

FIG. 17 is a plan view of another embodiment of a heat exchanger havinga figure 8 flow path in accordance with the present invention, withparts omitted for clarity of illustration;

FIG. 18 is an elevational view, partially in section, taken along theline 18--18 in FIG. 17;

FIG. 19 is a diagrammatic perspective view showing one side of the heatexchanger of FIG. 17, with parts omitted and broken away, and with thelongitudinal dimension expanded for purposes of clarity;

FIG. 20 is an elevational view, partially in section, of anotherembodiment of a heat exchanger having a zero loop flow path inaccordance with the present invention;

FIG. 21 is a general flow diagram for figure 8 flow path systems inaccordance with the present invention; and

FIG. 22 is a general flow diagram for zero loop flow path systems inaccordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 1 and 2, a heat exchanger system or unit 10 arrangedto employ figure 8 gas-solids transport techniques in accordance withthe present invention for recovery of heat from hot flue gases is shown.The heat exchanger 10 includes first and second operating or separationvessels 12 and 14 between which solid particulate material or sand 16 iscirculated for purposes of heat transfer with gaseous media.

The particulate material 16 is withdrawn from an inventory thereofmaintained within the vessel 12 through a downcomer 18 and pneumaticallytransferred to the vessel 14 through an associated lift line 20.Similarly, the vessel 14 is provided with a downcomer 22 communicatingwith an associated lift line 24 for purposes of transporting theparticulate material 16 from the vessel 14 to the vessel 12. The vessels12 and 14 are provided with cyclones 26 and 28 having dip legs 30 and 32for returning the particulate material separated by the cyclones to thedowncomers 18 and 22. The cyclones 26 and 28 may be mounted within theassociated vessels 12 and 14.

For purposes of illustration, the heat exchanger 10 is shown applied toa burner-furnace simulator 34 which generates the primary fluid or heatproviding stream herein comprising hot flue gases containing sufficientthermal energy to warrant heat recovery. In practice, the simulator 34may comprise any source of hot process gases such as flue gaspotentially containing a wide size range of particulate pollutants. Thelarger size particles of pollutants will typically be separated from theflue gas by the exchanger 10 and added to the inventory of particulatematerial. The temperature of the flue gas will generally be greater thanabout 700° F., and in the range of 1,000° F. to 3000° F. and higher fortypical process applications such as those involved in high temperatureprocess industries such as ceramics, glass making, metal melting orforging industries.

The heat exchanger unit 10 is fabricated of a suitable material such asaluminized steel, and the internal surfaces of the unit are free ofstructural irregularities, such as protruding metal edges or joints,which tend to disturb the smooth flow of the particulate material 16. Incases where particulate material temperatures in excess of 2,000° F. areencountered, refractory linings may be employed at locations of intenseheat and wear, such as in the cyclones.

The heat exchanger 10 may be arranged to provide preheated combustionair to the simulator 34 using the heat recovered from the simulator fluegases. To that end, a blower 36 directs the secondary fluid, whichcomprises ambient air, through the lift line 20 wherein the recoveredflue gas heat energy is used to elevate the ambient air temperature toprovide the preheated combustion air, which is then returned to thesimulator 34 by means of a preheated air return line (not shown). Theflow of the primary fluid or flue gases may be maintained or assisted bymeans of a suction fan (not shown) applied to vessel 12 to assurepneumatic conveyance in the lift line 24. It should be appreciated thatthe subject heat exchanger is not limited to the provision of preheatedcombustion air and that the secondary fluid or gaseous stream can beemployed in an entirely different process or for other purposes toprovide similar cost economies and conservation of energy.

The particulate material 16 comprises Ottawa Sand having a range ofspecific heats of 0.19 to 0.30 Btu/lb. F and having a particle size inthe range of from about 50 to about 420 microns. It should beappreciated that a wide variety of materials are suitable for use as aparticulate material in the subject techniques and systems. Typically,any nonsintering, inert, particulate material of suitable size forpurposes of pneumatic conveyance, phase separation and forming a loosepacked bed column thereof may be employed. In heat exchangeapplications, it is also desirable that the particulate material have ahigh specific heat, high hardness, high melting point, and high density.For example, glass beads having a specific gravity of 3.99 and a size inthe range from about 100 to about 149 microns have been foundsatisfactory. In selecting a particulate material, the attritioncharacteristics of the material itself and its erosion effects withrespect to the apparatus are considered parameters.

In the operation of the illustrated system, the sand 16, is withdrawnfrom the vessel 14 through the downcomer 22 and transferred to the liftline 24 through a laterally, substantially horizontally extending legportion 22a of the downcomer 22. The hot flue gases from the simulator34 pass upwardly through the lift line 24, pneumatically conveying withcocurrent heat exchange the relatively cool sand withdrawn from thevessel 14. The gas-solids stream, upon exiting from the lift line 24,impacts against a bonnet 40 which provides initial phase separation withsubstantial reduction of the velocity of the sand 16 relative to thegaseous medium and a further degree of heat exchange. In the separationof the solid and gaseous phases, the bonnet 40 cooperates with thevessel 12 to provide a settling chamber function, with the relativelyhot sand accumulating in the lower inventory portion of the vessel 12.

The cooled flue gases enter the cyclone 26 through a cyclone inletopening 26a, and any solids remaining in the flue gases are separatedtherefrom by the cyclone 26 and returned directly to the downcomer 18through cyclone dip leg 30. The dip leg 30 extends into the downcomer 18and the flowing loose packed bed of sand therein a sufficient radialdistance to avoid wall flow effects and assure the withdrawal of sandfrom the cyclone 26 through the dip leg and into the established flowregion within the loose packed bed. The cooled flue gases are ventedfrom the cyclone through a cyclone outlet and vent 42.

The hot sand 16 is withdrawn from the inventory thereof in the vessel 12and transferred to the lift line 20 through a horizontally extending legportion 18a of the downcomer 18. The hot sand is pneumatically conveyedupwardly through the lift line 20 by the ambient air flow provided bythe blower 36. The heat transfer process again occurs during pneumaticconveyance, and heat exchange is substantially completed by the time thesand and air exit from the lift line 20.

Upon exiting from the lift line 20, the gas-solids stream is impactedagainst a bonnet 44 for purposes of phase separation and further heattransfer in a manner similar to that occurring in the vessel 12.Accordingly, the now relatively cool sand 16 is collected in aninventory in the lower portion of the vessel 14 for purposes ofrecycling back to the vessel 12. The heated ambient air enters thecyclone 28 through a cyclone inlet opening 28a, and any remaining solidsare separated and returned directly to the downcomer 22 through thecyclone dip leg 32. The heated ambient air exits from the cyclone 28through a cyclone outlet and vent 46 which may be connected to apreheated air return line as noted above or directed to a separateprocess.

The dip leg 32 and downcomer 22 are connected in the same manner as thedip leg 30 and downcomer 18, and similarly assure withdrawal of sandfrom the cyclone 28. Further, the dip legs 30 and 32 are sealed againstreverse flow through the cyclones by the sand in the dip legs and in therespective downcomers during operation of the heat exchanger 10, as wellas during the initial start-up of the heat exchanger.

In accordance with the subject gas-solids transport techniques, the sand16 is withdrawn through the downcomers 18 and 22 in a bed or columnunder the influence of gravity and any pressure differences of the gasesat the opposite ends of the downcomers. As used herein, the term "loosepacked bed" or "loose packed bed column" contemplates a range ofconditions which can be established in particulate material under theinfluence of gravity wherein the void volume of the solids may be in therange of 30% to 40%.

In the operation of the heat exchanger system 10, either of thedowncomers 18 or 22 may be exposed to varying levels of aeration as aresult of operating pressure differentials. For example, the inletopening at the top of the downcomer 18 within the vessel 12 isessentially exposed to atmospheric pressure in the embodiment of FIG. 1,since the flue gas is directly vented, and the outlet opening adjacentthe end of the leg portion 18a of the downcomer 18 is exposed to thepressure in the lift line 20, which may be on the order of 2 psig.Accordingly, the downcomer 18 is exposed to a back pressure of 2 psigand corresponding levels of aeration.

The secondary fluid or gaseous medium in the lift line 20 tends to seepupwardly through the sand 16 in the downcomer 18 under the 2 psigdriving force, and a corresponding pressure head is developed in thecolumn of sand to effectively cancel out the 2 psig back pressure andleave only the forces of gravity unbalanced and acting on the sand.Thus, the downcomer is provided with sufficient height and designed toenable the column of sand therein when acted upon by gravity to developa pressure head at least equal to the difference of the gas pressures atopposite ends of the downcomer in the contemplated system. In thisregard, the density of the specific particulate material must beconsidered and optimization of design should reflect a substantiallyvertical orientation of the downcomer, since inclination of thedowncomer will reduce the potential pressure head which can bedeveloped. Further, the column of particulate material formed by thedowncomer should be of substantially uniform vertical configuration,since irregular shapes will also tend to reduce the maximum potentialpressure head.

In addition to the foregoing downcomer considerations to accommodateoperating pressure differentials, the downcomer is also designed toassure that the flow rates of particulate material therethrough aregreater than the rate of the upward seepage of gases. In the illustratedOttawa Sand system, the gas seepage rate through a loosely packed bed isabout 0.01 ft./sec. and, accordingly, the heat exchanger system 10 isdesigned to assure sand flow rates on the order of about 0.2 ft./sec.,or greater.

The foregoing discussion has been primarily concerned with the situationwherein the pressure differential tends to cause gas leakage in adirection opposite to that of the flow of the sand through thedowncomer. In some applications, the pressure differential tends tocause gas flow through the downcomer in the same direction as theparticulate material flow. For example, the downcomer 22 may be exposedto a back pressure which causes gas flow therethrough in the samedirection as the flow of the sand. In this instance, the density of theloose packed bed column within the downcomer 22 must be controlled inorder to assure that the sand 16 does not flow into the lift line 24solely as a result of the forces of gravity and the back pressure, aswell as to prevent the direct cross leakage of secondary fluid throughthe downcomer and into the operating vessel 12 through the lift line 24.To that end, the downcomer 22 and the loose packed bed column of sandtherein may be provided with a sufficient height to assure a cancellingpressure drop effect or, alternatively, an enlarged diameter portion 48as shown in phantom outline in FIG. 1 may be provided to increase thedensity of the bed by decreasing the sand flow rate through the enlargedportion 48. Accordingly, this downcomer is also designed to assure thatgravity comprises the only unbalanced force acting to withdraw sand fromthe operating vessel 14 and to assure that no direct cross leakage ofgas occurs.

A relatively low level of entrainment of fluids or gaseous media by thesand so as to result in the cross flow of flue gas and air to oppositesides of the exchanger has been found in the normal range of operationof the heat exchanger 10. The bulk density of the sand within thedowncomers at typical air, flue gas and sand flow rates will result inabout a 0.02 mol % crossover flow between the sides of the exchanger atatmospheric pressure. Accordingly, the loose packed beds in thedowncomers provide effective dynamic seals with respect to fluidentrainment as well as bulk leakage tending to result from operatingpressure differences.

As indicated above, the sand 16 is withdrawn from the vessels 12 and 14through the downcomers 18 and 22 under the influence of gravity and in agravity feed-lock system. More particularly, the sand accumulates ineach of the leg portions 18a and 22a, assuming natural angles of reposereflecting the pressure differentials applied to the downcomers, and itdoes not flow into the associated lift lines 20,24 in the absence ofpositive feed bias. The accumulation of sand is shown with reference tothe downcomer 18 in FIG. 3, it being understood that the sameaccumulation condition occurs in the downcomer 22.

As shown in FIG. 3, the sand 16 accumulates in the leg portion 18a ofthe downcomer 18 and assumes a natural angle of repose R in the absenceof positive feed bias. The length of the horizontal leg portion 18a isjust long enough to prevent spillover or weeping of the sand into thelift line 20 solely due to the forces of gravity and the operatingpressure differentials imposed upon the downcomer 18. A positive feedbias is obtained by disturbing the sand from its natural angle of reposeR by the controlled flow of an inert gas, such as trigger air introducedthrough line 50, to cause the sand to assume an operating feed angle Raand directly spillover into the lift line 20. In this manner, a gravityfeed-lock system is provided, and the withdrawal and feeding of the sandare achieved with a minimum power requirement and without the use ofvalves and actuators therefor.

The required length of the leg portion 18a of the downcomer will vary asa function of the characteristics of the particulate material, thespecific configuration and orientation of the leg portion, and theoperating pressure differentials of the system. In order to accommodatesystem variations and to avoid undesirably long leg portions, the crosssectional area of the leg portion can be reduced per se or effectivelyreduced by the use of an adjustable weir 52, as shown in the embodimentof FIG. 4. For purposes of convenience, the elements of FIG. 4 areidentified with the corresponding reference numerals used in FIG. 3 andthe addition of a prime designation. The weir 52 is arranged for radialmovement through the leg portion 18a' to adjustably reduce the crosssectional area of the leg portion and cause the accumulation of sand toterminate in its natural angle of repose R' with a reduced horizontaldimension or lateral extent. As shown in FIG. 4, the angle of repose R'is substantially equal to the angle R, but the weir 52 causes theaccumulation of sand to terminate in a shorter horizontal distance.

As discussed in greater detail below, the particulate material or sand16 withdrawn through each of the downcomers is transferred to theassociated lift lines and introduced therein to maximize the velocitydifference between the particulate material and the gaseous conveyingstream. In addition, it has been found advantageous to tangentiallyintroduce the sand 16 into the lift line 24 for pneumatic transfer andconvenyance by the hot flue gases. As shown in FIG. 5, the tangentialintroduction of the sand 16 causes it to sweep about the insideperiphery of the lift line 24 to form a heat insulating layer of sand16a which tends to structurally protect the lift line from the extremelyhigh flue gas temperatures in a mixing zone 24a adjacent the outlet ofthe leg portion 22a. In the lift line 20, radial introduction of thesand 16 as shown in FIG. 3 has been found satisfactory and to provide amixing zone 20a adjacent the outlet of the leg portion 18a.

As the sand enters the lift lines and the mixing zones 20a and 24a, itinitially falls downwardly a short distance as it is violently mixedwith the pneumatic conveying fluid. The violent mixing of the sand withthe air in the mixing zone 20a is described below in detail withreference to FIG. 3, it being understood that similar violent mixingoccurs in the mixing zone 24a with the addition of the heat insulatinglayer 16a resulting from tangentially introducing the sand.

Referring to FIG. 3, the sand 16 is biased from its natural angle ofrepose in the leg portion 18a and into the lift line 20 at the beginningof the pneumatic conveyance step. The sand initially clouds and falls orfloats downward in the mixing zone 20a. The direction of the sand flowis then reversed by the gaseous conveying medium, and the sand israpidly accelerated, with turbulent flow conditions prevailing as thesand particles are swept upwardly in a slipstream fashion. The violentmixing of the particulate material with the gaseous stream ischaracterized by turbulent flow conditions including random movement andrepeated acceleration and deceleration of the sand appearing to resultfrom eddies and interparticle collisions within the gaseous stream. Inaccordance with visual observations, the mixing zone 20a extends withinthe lift line for a distance of several feet, depending upon the weightratio of sand to gaseous medium and the velocity of the gaseous medium.

The flow conditions in the mixing zone 20a have been found to maximizethe rate of heat transfer between the sand and gaseous medium. Themaximization of the rate of heat transfer is associated with relativelyhigh slip velocities existing in the zone 20a and reflects largeincreases in the coefficient of heat transfer.

The coefficient of heat transfer is again maximized at the end of thepneumatic conveyance steps by directing the sand and gaseous streamexiting from each of the lift lines 20 and 24 into the associatedbonnets 44 and 40 to provide further violent mixing. This is illustratedin FIG. 6 for the lift line 20 and bonnet 44 where a mixing zone 44a isshown, it being appreciated that a similar mixing zone 40a (not shown)is provided with respect to the lift line 24 and bonnet 40.

The mixing zone 44a is formed adjacent a downwardly opening chamber 45defined by the bonnet 44. A dense cloud of sand-gaseous medium isbelieved to be continuously entrapped within the chamber 45 and toprotect the bonnet from erosion and to substantially decelerate the sandparticles parimarily as a result of interparticle collisions. The flowconditions within mixing zone 44a have been found to maximize thecoefficient of heat transfer in a manner similar to that in the mixingzone 20a.

Referring to FIG. 7, an automatic feed control arrangement 54 forpurposes of regulating the withdrawal of sand 16 through the downcomer18 as a function of the pressure drop in the associated lift line 20 isillustrated. A differential pressure sensor 56 is connected across thelift line 20 by means of lines 58 and 60 to respectively measure thepressure of the secondary fluid prior to the introduction of sand and atthe exit of the lift line in order to determine the pressure dropthrough the lift line. The sensor 56 is connected by line 62 to a flowcontrol valve 64, which in turn is connected to a trigger air source 66by line 68. If the sensed pressure drop across the lift line 20 is lowerthan a preselected value, the flow control valve 64 is actuated toprovide increased trigger air through the line 50.

The trigger air is introduced into the accumulated sand 16 adjacent thelower portion of the downcomer 18 through a nozzle 50a (FIG. 3) to biasthe accumulated sand from its natural angle of repose in the leg portion18a and into the lift line 20 for pneumatic conveyance and heatexchange. If the sensed pressure drop is greater than the preselectedvalue, the flow control valve 64 closes somewhat to reduce the flow oftrigger air and the sand will assume a substantially slower flow rate inthe downcomer 18. For purposes of adjusting the pressure drop, thesensor 56 includes a set point control knob 56a.

In the control arrangement 54, the monitored pressure drop is across thelift line 20 and advantageously enables prevention of sand flow ratescorresponding to the choking velocity in the lift line 20 under theparticular operating conditions. However, any operating pressure dropcan be monitored, including, for example, combined pressure dropsthrough the lift line 20, the cyclone 28, and a downstream process inwhich the secondary fluid is employed. It is advantageous to include thepressure drop in the lift line 20 in the monitored pressure drop, sinceit represents the largest pressure drop in the heat exchanger system 10and readily enables avoidance of choking velocities.

The withdrawal of sand 16 through the downcomer 18 may also be regulatedas a function of the outlet temperature of the secondary fluid or atemperature of the system to which the exchanger is applied.Accordingly, the feed control arrangement 54 may be modified by thesubstitution of a temperature sensor 63 for the differential pressuresensor 56 or the two sensors 56,63 may be employed in a combined system,as discussed below in detail.

The temperature sensor 63 is arranged to monitor the secondary fluidoutlet temperature at a location downstream of the lift line 20 by meansof line 65 and to transmit an outlet temperature control signal throughline 67 to the flow control valve 64. The outlet temperature is directlyrelated to the sand flow rate, and the flow control valve 64 istemperature modulated to provide a corresponding level of trigger airbias to the accumulated sand 16 in the leg portion 18a in accordancewith the difference between the sensed outlet temperature and thedesired temperature as set by adjustment of a control knob 63a.

As indicated above, it is also convenient to combine the temperature andpressure controls in a system wherein the temperature sensor 63 isarranged to control the flow rate of the sand in accordance with asensed outlet temperature, and the pressure sensor 56 is set to operateat a sand flow rate below the choking velocity under the operatingconditions in the lift line 20. Thus, the combined system is essentiallytemperature controlled, with pressure drop regulation governing only toprevent choking in the lift line.

On the opposite side of the heat exchanger system 10, the withdrawal ofthe sand 16 from the vessel 14 through the downcomer 22 is controlled ina following manner relative to the sand flow rate resulting from thepreselected pressure drop and/or outlet temperature as discussed above.More particularly, an automatic level control arrangement 70, shown inFIG. 8, maintains a constant inventory of sand 16 in the vessel 14 abovethe inlet opening of the downcomer 22. Accordingly, the sand flow ratethrough the downcomer 22 will be about equal to the sand flow ratethrough the downcomer 18, since the total amount of sand in the systemis substantially constant.

In the control arrangement 70, an air supply source 72 and a regulator74 are used to introduce a constant gas flow into the vessel 14 at thedesired level of the inventory of sand 16 through a line 76 and arestricted nozzle 78. The gas pressure in the line 76 is compared withthe gas pressure in the vessel 14 by means of a differential pressuresensor 80 connected therebetween by means of lines 82 and 84. The sensor80 is connected by line 86 to a flow control valve 88. In accordancewith the sensed pressure differential, the valve 88 is actuated toprovide trigger air from a trigger air source 90, which is connected tothe valve 88 by a line 92, to the downcomer 22 through a line 94 to biasthe accumulated particulate material into the lift line 24.

In the operation of the level control arrangement 70, if the level ofthe inventory of sand 16 is below the level of the nozzle 78, arelatively small pressure drop will be sensed by the differentialpressure sensor 80, and the flow control valve 88 will be closed to stopthe flow of trigger air and the withdrawal of sand. The sand 16 willthen accumulate in the downcomer 22 and vessel 14 until it reaches thedesired inventory level and covers the nozzle 78. When the nozzle 78 iscovered with sand, the sensed pressure differential will increase, andthe flow control valve 88 will be actuated to cause the flow of triggerair through the line 94 to bias the accumulated sand 16 from its naturalangle of repose in the leg portion 22a into the lift line 24.

It should be appreciated that the control arrangements 54 and 70 can beapplied to the opposite sides of the heat exchanger system 10. Moreparticularly, the control arrangement 54 may be used to regulate therates of withdrawal of sand through the downcomer 22 from the vessel 14,and the control arrangement 70 may be used to regulate the inventory ofsand in the vessel 12.

Referring to FIG. 9, the heat exchanger system 10 is diagrammaticallyshown for purposes of illustrating various temperature and pressuremeasurements used in the subsequent explanation of the system. Asindicated in FIG. 9, the secondary fluid may be fed to the simulator 34when the system is operated in a coupled mode, or the secondary fluidmay be directed to a separate and different process, such as, a reactorsystem or a simple space heater application. Further, a portion of theheated secondary fluid may be returned to the simulator 34 as preheatedcombustion air, and the remaining portion of the secondary fluid may beused in a separate process. The temperature and pressure measurementsare itemized below, and the notation employed in the subsequent analysisof data is summarized thereafter. In the following discussion andtables, all temperatures are in degrees Fahrenheit and pressures inmillimeters of water unless otherwise indicated.

Temperature Measurements

t_(a1) --Secondary fluid or air inlet temperature to heat exchanger.

t_(a4) --Temperature of secondary fluid in lift line 20 at a distance ofabout 2 feet above the outlet of leg portion 18a.

t_(a5) --Temperature of secondary fluid in lift line 20 at a distance ofabout 7 feet above the outlet of leg portion 18a.

t_(a2) --Secondary fluid outlet temperature as it leaves vessel 14.

t_(g1) --Primary fluid or flue gas inlet temperature.

t_(g8) --Temperature of primary fluid in lift line 24 at a distance ofabout 2 feet above the outlet of leg portion 22a.

t_(g7) --Temperature of primary fluid in lift line 24, at a distance ofabout 7 feet above the outlet of leg portion 22a.

t_(g2) --Primary fluid outlet temperature as it leaves vessel 12.

t_(g12) --Simulator combustion air inlet temperature.

t_(s2) --Temperature of sand into lift line 24.

t_(s3) --Temperature of sand into lift line 20.

Pressure Measurements

P₁ --Pressure of secondary fluid or air into lift line 20.

P₂ --Pressure of secondary fluid as it leaves vessel 14.

P₃ --Pressure of primary fluid or flue gas into lift line 24.

P₄ --Pressure of primary fluid as it leaves vessel 12.

ΔPa--Pressure drop across lift line 20.

ΔPg--Pressure drop across lift line 24.

Calculated Values

a--Flow rate of secondary fluid or air through lift line 20 in lbs./hr.

g--Flow rate of primary fluid or flue gas through lift line 24 inlbs./hr.

s--Flow rate of solid particulate material or sand in lbs./hr.

Qa--Heat absorbed by secondary fluid in Btu/hr.

Qg--Heat lost by primary fluid in Btu/hr. ##EQU1## Alpha a, the weightratio of the sand flow rate s to the secondary fluid or air flow rate a,adjusted by their respective specific heats c_(s) and c_(a) forconvenience in mathematical expressions of heat transfer relationships,and essentially representing the sand loading of the air stream.##EQU2## Alpha g, the weight ratio as defined above for α_(a) but withrespect to the primary fluid or flue gas flow rate g. ##EQU3## Beta, theweight ratio of the secondary fluid or air flow rate a to the primaryfluid or flue gas flow rate g, adjusted by the specific heats asindicated, and essentially representing the weight flow rate ratio ofthe fluid streams.

Hr The percent heat recovery defined as the percentage obtained bydividing the heat absorbed by the secondary fluid, Qa, by the totalamount of heat theoretically available from the primary fluid, assumingthat its inlet temperature t_(g1) is reduced to the inlet temperaturet_(a1) of the secondary fluid.

Hr* The theoretical percent heat recovery defined as the percentageobtained by dividing the quantity of heat which could be absorbed by thesecondary fluid if ideal cocurrent heat exchange occurred by the totalamount of heat theoretically available from the primary fluid if it werecooled to the inlet temperature t_(a1).

E The efficiency of the exchanger defined as the percentage obtained bydividing Hr by Hr*, and essentially comprising a comparison of theactual performance of the cocurrent heat exchanger with the maximumtheoretical heat recovery possible under conditions of ideal cocurrentheat transfer.

The operation of the heat exchanger 10 in a coupled mode is illustratedby the test runs reported in Table I. In these runs, the secondary fluidwas ambient air which was returned to the simulator 34 as preheatedcombustion air. The Ottawa Sand employed in these runs had a particlesize of about 240 microns. The heat exchanger was controlled bymonitoring the pressure drop ΔPa in the lift line 20 with the automaticfeed control arrangements 54 and 70 as described above. In each of theruns, a predetermined pressure drop across the lift line 20 was set andthe system was allowed to reach steady state conditions with theachievement of the preselected pressure drop.

The air and flue gas flow rates reported in Table I result in feedvelocities to the air lift line 20 in the range of 20 to 30 feet persecond and to the flue gas lift line 24 in the range of 80 to 120 feetper second. In the air lift line, the exiting velocity will be abouttwice the feed velocity due to the increase in temperature of the airand, similarly, the exiting flue gas velocity will be about one-half thefeed velocity due to the cooling of the flue gas.

                                      TABLE 1                                     __________________________________________________________________________    Run                                                                           No.                                                                              a  g  s  Δpa                                                                         Δpg                                                                         α.sub.a                                                                    α.sub.g                                                                    β                                                                           Qa.sup.1                                                                         Qg.sup.1                                                                         Hr Hr*                                                                              E                                    __________________________________________________________________________    1  1888                                                                             1914                                                                             7127                                                                             180 240 4.40                                                                             4.00                                                                             0.91                                                                             310                                                                              346                                                                              0.44                                                                             0.43                                                                             1.02                                 2  1888                                                                             1906                                                                             8346                                                                             200 240 4.85                                                                             4.52                                                                             0.93                                                                             225                                                                              252                                                                              0.41                                                                             0.44                                                                             0.94                                 3  1888                                                                             1916                                                                             3455                                                                             75  140 2.14                                                                             1.92                                                                             0.90                                                                             291                                                                              336                                                                              0.37                                                                             0.38                                                                             0.97                                 4  1888                                                                             1917                                                                             7924                                                                             170 220 4.75                                                                             4.35                                                                             0.92                                                                             266                                                                              296                                                                              0.41                                                                             0.43                                                                             0.95                                 5  1888                                                                             1910                                                                             3252                                                                             60  120 1.92                                                                             1.75                                                                             0.91                                                                             233                                                                              265                                                                              0.36                                                                             0.38                                                                             0.96                                 6  1888                                                                             1909                                                                             5723                                                                             140 210 3.37                                                                             3.10                                                                             0.92                                                                             252                                                                              284                                                                              0.40                                                                             0.41                                                                             0.96                                 7  1888                                                                             1917                                                                             3396                                                                             72  140 2.10                                                                             1.88                                                                             0.90                                                                             295                                                                              345                                                                              0.37                                                                             0.38                                                                             0.97                                 8  1888                                                                             1916                                                                             4143                                                                             80  160 2.56                                                                             2.30                                                                             0.90                                                                             300                                                                              352                                                                              0.37                                                                             0.39                                                                             0.95                                 9  1888                                                                             1914                                                                             7361                                                                             180 220 4.53                                                                             4.10                                                                             0.91                                                                             320                                                                              369                                                                              0.40                                                                             0.43                                                                             0.94                                 10 1957                                                                             1985                                                                             3281                                                                             70  140 1.97                                                                             1.77                                                                             0.90                                                                             276                                                                              322                                                                              0.36                                                                             0.42                                                                             0.84                                 11 1957                                                                             1984                                                                             3925                                                                             100 160 2.35                                                                             2.12                                                                             0.90                                                                             296                                                                              333                                                                              0.38                                                                             0.39                                                                             0.98                                 12 1957                                                                             1983                                                                             6710                                                                             160 240 4.00                                                                             3.63                                                                             0.91                                                                             311                                                                              354                                                                              0.40                                                                             0.42                                                                             0.95                                 13 1634                                                                             1655                                                                             2708                                                                             98  95  1.88                                                                             1.72                                                                             0.91                                                                             197                                                                              230                                                                              0.35                                                                             0.37                                                                             0.95                                 14 1634                                                                             1655                                                                             3058                                                                             100 95  2.13                                                                             1.94                                                                             0.91                                                                             201                                                                              234                                                                              0.36                                                                             0.38                                                                             0.94                                 15 1634                                                                             1655                                                                             4244                                                                             115 120 2.94                                                                             2.69                                                                             0.91                                                                             214                                                                              248                                                                              0.37                                                                             0.41                                                                             0.92                                 16 1619                                                                             1638                                                                             8310                                                                             240 240 5.80                                                                             5.33                                                                             0.92                                                                             233                                                                              266                                                                              0.41                                                                             0.44                                                                             0.93                                 17 1310                                                                             1329                                                                             5728                                                                             240 280 4.92                                                                             4.47                                                                             0.91                                                                             209                                                                              244                                                                              0.40                                                                             0.43                                                                             0.92                                 18 1310                                                                             1329                                                                             3149                                                                             120 120 2.81                                                                             2.54                                                                             0.91                                                                             192                                                                              215                                                                              0.38                                                                             0.40                                                                             0.94                                 19 1310                                                                             1330                                                                             2277                                                                             80  80  2.04                                                                             1.83                                                                             0.90                                                                             185                                                                              216                                                                              0.35                                                                             0.38                                                                             0.94                                 20 1310                                                                             1330                                                                             1774                                                                             40  40  1.59                                                                             1.42                                                                             0.90                                                                             175                                                                              210                                                                              0.33                                                                             0.35                                                                             0.93                                 21 1573                                                                             1593                                                                             3202                                                                             60  100 2.35                                                                             2.14                                                                             0.91                                                                             210                                                                              235                                                                              0.37                                                                             0.39                                                                             0.95                                 22 1573                                                                             1598                                                                             1892                                                                             30  80  1.41                                                                             1.26                                                                             0.89                                                                             214                                                                              249                                                                              0.33                                                                             0.34                                                                             0.95                                 23 1573                                                                             1595                                                                             3781                                                                             130 120 2.81                                                                             2.54                                                                             0.90                                                                             230                                                                              276                                                                              0.37                                                                             0.40                                                                             0.92                                 24 1573                                                                             1595                                                                             7203                                                                             240 240 5.33                                                                             4.84                                                                             0.91                                                                             251                                                                              302                                                                              0.39                                                                             0.43                                                                             0.90                                 25 1630                                                                             1662                                                                             6813                                                                             240 320 5.06                                                                             4.49                                                                             0.89                                                                             353                                                                              394                                                                              0.41                                                                             0.43                                                                             0.97                                 26 1665                                                                             1697                                                                             5182                                                                             180 180 3.77                                                                             3.34                                                                             0.89                                                                             348                                                                              388                                                                              0.40                                                                             0.41                                                                             0.97                                 27 1665                                                                             1697                                                                             4269                                                                             90  140 3.12                                                                             2.75                                                                             0.88                                                                             339                                                                              378                                                                              0.39                                                                             0.40                                                                             0.98                                 28 1665                                                                             1697                                                                             3335                                                                             65  140 2.44                                                                             2.15                                                                             0.88                                                                             325                                                                              365                                                                              0.38                                                                             0.38                                                                             0.98                                 29 1179                                                                             1203                                                                             6785                                                                             260 220 6.96                                                                             6.18                                                                             0.89                                                                             259                                                                              295                                                                              0.42                                                                             0.44                                                                             0.95                                 30 1179                                                                             1203                                                                             5108                                                                             200 180 5.25                                                                             4.66                                                                             0.89                                                                             249                                                                              281                                                                              0.41                                                                             0.43                                                                             0.96                                 31 1179                                                                             1203                                                                             4918                                                                             180 130 5.06                                                                             4.49                                                                             0.89                                                                             240                                                                              281                                                                              0.40                                                                             0.43                                                                             0.93                                 32 1179                                                                             1203                                                                             3151                                                                             140 100 3.25                                                                             2.87                                                                             0.88                                                                             231                                                                              261                                                                              0.39                                                                             0.40                                                                             0.96                                 33 1179                                                                             1203                                                                             2341                                                                             120 85  2.42                                                                             2.13                                                                             0.88                                                                             221                                                                              255                                                                              0.37                                                                             0.38                                                                             0.96                                 34 1179                                                                             1203                                                                             1851                                                                             100 70  1.92                                                                             1.68                                                                             0.88                                                                             209                                                                              245                                                                              0.35                                                                             0.37                                                                             0.96                                 35 1179                                                                             1203                                                                             1624                                                                             75  70  1.68                                                                             1.47                                                                             0.88                                                                             203                                                                              242                                                                              0.34                                                                             0.35                                                                             0.95                                 36 1179                                                                             1203                                                                             1432                                                                             60  55  1.49                                                                             1.30                                                                             0.87                                                                             188                                                                              235                                                                              0.31                                                                             0.35                                                                             0.89                                 37 1989                                                                             2025                                                                             8611                                                                             230 280 5.25                                                                             4.67                                                                             0.89                                                                             420                                                                              461                                                                              0.42                                                                             0.43                                                                             0.97                                 38 1989                                                                             2025                                                                             7070                                                                             200 240 4.32                                                                             3.83                                                                             0.89                                                                             410                                                                              450                                                                              0.41                                                                             0.42                                                                             0.97                                 39 1989                                                                             2025                                                                             5616                                                                             160 180 3.44                                                                             3.05                                                                             0.89                                                                             394                                                                              433                                                                              0.40                                                                             0.41                                                                             0.97                                 40 1989                                                                             2027                                                                             4491                                                                             150 180 2.75                                                                             2.42                                                                             0.88                                                                             383                                                                              452                                                                              0.37                                                                             0.41                                                                             0.97                                 __________________________________________________________________________     .sup.1 Number reported in thousands                                      

In considering the test runs of Table I, reference is made to curves Aand B in FIG. 10 which respectively show the relationship between thepressure drop ΔPa and the sand flow rate for run Nos. 1 to 9 having anair flow rate of 1,888 lbs./hr. and run Nos. 29 to 36 having an air flowrate of 1,179 lbs./hr. As shown in FIG. 10, the pressure drop ΔPa isdirectly related to the sand flow rate with sufficient sensitivity toenable the later to be controlled by pressure drop regulation, and thepressure drop is inversely related to the absolute value of the air flowrate. The inverse relationship between the pressure drop and theabsolute value of the air flow rate effectively relates the pressuredrop to the sand loading of the air stream α_(a) over reasonable rangesof operation of the heat exchanger. Similar relationships between thepressure drop and the sand and flue gas flow rates have been found tooccur in the flue gas side of the exchanger, although the pressure dropΔPg appears to be less sensitive to variations of the absolute value ofthe flue gas flow rate and sand loading α_(g). The difference inpressure drop response on each side of the heat exchanger is believed tobe primarily associated with the acceleration of the air as it is heatedduring pneumatic conveyance and the deceleration of the flue gas as itis cooled. However, the inverse relationship is observed in all casesand the α_(a),α_(g) values are indicative of the total pressure drop ofthe heat exchanger.

The inverse relationship between pressure drop and the absolute valuesof the air and flue gas flow rates is a significant factor in the heattransfer techniques herein and the variable area operation of the heatexchanger as further discussed below. However, it should be appreciatedthat the pressure drop at a constant air or flue gas flow rate issubstantially a function of the sand flow rate and, more particularly,the energy required to accelerate the sand during pneumatic conveyance,the lift energy to convey the sand from the bottom of the lift line intothe vessel, the acceleration of the air as it is heated and thedeceleration of the flue gas as it is cooled as they each flow throughtheir respective lift lines, and the friction losses during pneumaticconveyance. The relative pressure drop contributions of these factorswill vary depending upon the particular flow conditions, but theacceleration of the sand is a most significant factor and tends tomaximize the rate of heat transfer, as previously discussed. Theacceleration of the sand primarily occurs in the mixing zones for theindicated conditions of pneumatic conveyance, and it has also beenobserved in other portions of the lift line. For example, the formationof sand cloud layers has been observed above the level of the mixingzone 20a in the lift line 20. These cloud layers flow upwardly anddownwardly at the interior wall surface of the lift line 20 and they areeventually picked up by the gaseous stream and moved upwardly.

The overall quantity of heat transferred between the primary andsecondary fluids at constant air and flue gas flow rates will increasewith increasing sand flow rates as reflected by the percent heatrecovery values reported in Table I. For example, run Nos. 29 to 36 showthat the percent heat recovery increases from 31% to 42% as the sandflow rate increases. The increasing sand flow rate will result incorresponding increases in pressure drop since the sand flow rate is thedominant factor in pressure drop. As the sand flow rate is increased tothe upper practical limit of operation for which the heat exchanger isdesigned at the given air and flue gas flow rates, the correspondingincrease in the quantity of heat transferred becomes less. This is shownby comparing the percent heat recovery values for runs Nos. 29 to 32with those of run Nos. 33 to 36.

In the coupled mode of operation, the decreasing quantity of heattransferred as the sand flow rate approaches the design limits of theheat exchanger 10 is conveniently illustrated by consideration of thesensitivities of the air and flue gas outlet temperatures t_(a2) andt_(g2) to variation of the sand flow rate. Assuming ideal cocurrent heatexchange wherein the temperature of the sand and conveying fluid becomeequal during the pneumatic conveying steps, a heat balance of theexchanger 10 enables the ideal outlet temperatures t_(a2) and t_(g2) tobe expressed in the following equations as functions of the inlet airand flue gas temperatures t_(a1), t_(g1) and the sand loadings α_(a),α_(g). ##EQU4## The derivative of each of the foregoing equations withrespect to the sand flow rate results in the following equations whichexpress the sensitivities of t_(a2) and t_(g2) to the sand flow.##EQU5## The sensitivity relationships for the runs of Table I areillustrated in FIG. 11 wherein the calculated sensitivities as definedin equations (3) and (4) are shown as a function of the sand flow rate.As shown in FIG. 11, the sensitivities or rates of change of the air andflue gas outlet temperatures decrease with increasing sand flow rates.Thus, the outlet temperatures are more responsive to variations in thesand flow rate in the lower range of sand flow rates. As the sand flowrate approaches the upper design limit for the particular system, onlynominal further changes in the outlet temperatures are obtained. Thequantity of heat transferred will follow the changes in the outlettemperatures since substantially equal air and flue gas flow rates existin the coupled mode of operation.

The foregoing relationships are further illustrated by consideration ofthe operation of the heat exchanger in terms of percent heat recoveryand the total heat exchanger pressure drop as represented by the sum ofthe pressure drops in the lift lines 20 and 24. Referring to FIG. 12,the relationship between the percent heat recovery Hr and the total heatexchanger lift line pressure drops, ΔPa plus ΔPg, is shown for the runsof Table I. In this instance, a single curve for all of the runs ofTable I is appropriate since the variations in the absolute values ofthe air and flue gas flow rates are reflected in the total pressuredrop.

It is apparent from FIG. 12 that the percent heat recovery is notsubstantially reduced by significant reductions in the pressure dropwhich directly reflects the power requirements of the system. Forexample, assuming operation at a pressure drop of 400 mm of water, thepercent heat recovery is about 40%. If the pressure drop and powerrequirement are reduced by 50% by decreasing the sand flow rate, thereduction in the percent heat recovery is only about 4% to an Hr valueof 36%. The pressure drop reductions and power requirement benefits tobe obtained by further reductions in the sand flow rate become morecostly in terms of percent heat recovery as the air and flue gas flowrates become more dominant in the system pressure drop at relatively lowsand flow rates. However, it is apparent that the heat exchanger isoperable over a significant range of conditions with acceptablevariations in percent heat recovery. Thus, the power requirements forthe heat exchanger can be minimized with only a minor decrease in thetotal amount of heat recovered in accordance with variable areaoperation of the heat exchanger. It should be appreciated that thedecrease in the power requirement and sand flow rate will result in adecrease in the air outlet temperature and an increase in the outlettemperature of the vented flue gas as indicated in FIG. 11.

The operation of the heat exchanger system 10 in an uncoupled mode, withthe primary fluid or flue gas and the secondary fluid or air both beingvented to the atmosphere, is illustrated by the test runs reported belowin Table II. The Ottawa Sand employed in these runs had a particle sizeof about 420 microns. In all cases, the heat exchanger system 10 wascontrolled by the automatic control arrangements 54 and 70, and thesystem was allowed to reach steady state operating conditions with theachievement of the selected pressure drop ΔPa at the preset air and fluegas flow rates.

The indicated air flow rates resulted in air velocities into the liftline 20 ranging from 20 to 65 feet per second and exiting velocitiesranging from 40 to 130 feet per second. The flue gas velocities into thelift line 24 ranged from 75 to 155 feet per second and the exitingvelocities were 40 to 80 feet per second.

                                      TABLE II                                    __________________________________________________________________________    Run                                                                           No.                                                                              a  g  s   Δpa                                                                        Δpg                                                                        α.sub.a                                                                    α.sub.g                                                                    β                                                                           Qa.sup.1                                                                         Qg.sup.1                                                                         Hr Hr*                                                                              E                                     __________________________________________________________________________    41 3543                                                                             1517                                                                             22542                                                                             400                                                                              480                                                                              7.10                                                                             15.14                                                                            2.30                                                                             467                                                                              463                                                                              0.66                                                                             0.65                                                                             1.02                                  42 2994                                                                             1517                                                                             22339                                                                             440                                                                              530                                                                              8.46                                                                             15.20                                                                            1.80                                                                             422                                                                              457                                                                              0.50                                                                             0.62                                                                             0.81                                  43 2286                                                                             1538                                                                             10735                                                                             400                                                                              380                                                                              5.40                                                                             7.30                                                                             1.35                                                                             357                                                                              413                                                                              0.50                                                                             0.53                                                                             0.94                                  44 3050                                                                             1558                                                                             10847                                                                             400                                                                              370                                                                              4.04                                                                             7.17                                                                             1.77                                                                             413                                                                              440                                                                              0.57                                                                             0.59                                                                             0.97                                  45 3785                                                                             1558                                                                             10311                                                                             280                                                                              340                                                                              3.00                                                                             6.60                                                                             2.20                                                                             432                                                                              464                                                                              0.60                                                                             0.62                                                                             0.96                                  46 3798                                                                             1791                                                                             10862                                                                             260                                                                              360                                                                              3.15                                                                             6.10                                                                             1.94                                                                             424                                                                              462                                                                              0.56                                                                             0.60                                                                             0.95                                  47 2985                                                                             1936                                                                             16186                                                                             410                                                                              360                                                                              6.04                                                                             8.53                                                                             1.41                                                                             420                                                                              475                                                                              0.52                                                                             0.55                                                                             0.94                                  48 2152                                                                             1949                                                                             11350                                                                             390                                                                              300                                                                              6.05                                                                             6.12                                                                             1.01                                                                             350                                                                              433                                                                              0.43                                                                             0.46                                                                             0.92                                  49 4091                                                                             1949                                                                              9417                                                                             240                                                                              280                                                                              2.54                                                                             4.85                                                                             1.91                                                                             446                                                                              475                                                                              0.55                                                                             0.58                                                                             0.95                                  50 3707                                                                             1188                                                                             19531                                                                             440                                                                              680                                                                              5.60                                                                             15.71                                                                            2.81                                                                             399                                                                              351                                                                              0.71                                                                             0.70                                                                             1.00                                  51 3053                                                                             1188                                                                             21015                                                                             400                                                                              610                                                                              7.56                                                                             17.64                                                                            2.33                                                                             390                                                                              405                                                                              0.67                                                                             0.67                                                                             1.00                                  52 3161                                                                             1214                                                                             13915                                                                             310                                                                              420                                                                              4.84                                                                             11.44                                                                            2.36                                                                             388                                                                              402                                                                              0.66                                                                             0.66                                                                             1.00                                  53 4017                                                                             1214                                                                             10943                                                                             260                                                                              360                                                                              2.96                                                                             8.85                                                                             2.99                                                                             402                                                                              411                                                                              0.69                                                                             0.69                                                                             0.99                                  54 2123                                                                             1520                                                                             10017                                                                             330                                                                              280                                                                              5.44                                                                             6.88                                                                             1.26                                                                             307                                                                              452                                                                              0.42                                                                             0.52                                                                             0.81                                  55 1416                                                                             1256                                                                             10072                                                                             380                                                                              260                                                                              8.40                                                                             8.61                                                                             1.02                                                                             267                                                                              331                                                                              0.44                                                                             0.46                                                                             0.94                                  56 1526                                                                             1256                                                                              5366                                                                             210                                                                              150                                                                              4.19                                                                             4.58                                                                             1.09                                                                             268                                                                              323                                                                              0.44                                                                             0.47                                                                             0.95                                  57 1521                                                                             1256                                                                              4756                                                                             200                                                                               95                                                                              3.76                                                                             4.09                                                                             1.09                                                                             257                                                                              316                                                                              0.43                                                                             0.46                                                                             0.92                                  58 1517                                                                             1256                                                                              3844                                                                             160                                                                               80                                                                              2.96                                                                             3.22                                                                             1.09                                                                             247                                                                              310                                                                              0.41                                                                             0.45                                                                             0.91                                  59 1535                                                                             1256                                                                              3277                                                                             140                                                                               60                                                                              2.50                                                                             2.75                                                                             1.10                                                                             240                                                                              305                                                                              0.40                                                                             0.44                                                                             0.90                                  60 1457                                                                             1519                                                                             11217                                                                             380                                                                              240                                                                              9.22                                                                             8.04                                                                             0.87                                                                             297                                                                              381                                                                              0.40                                                                             0.44                                                                             0.92                                  61 1980                                                                             1844                                                                             10084                                                                             360                                                                              220                                                                              5.94                                                                             5.83                                                                             0.98                                                                             347                                                                              419                                                                              0.43                                                                             0.46                                                                             0.94                                  62 1139                                                                             1157                                                                              7065                                                                             390                                                                              280                                                                              7.32                                                                             6.57                                                                             0.90                                                                             219                                                                              286                                                                              0.40                                                                             0.44                                                                             0.92                                  63 1203                                                                             1156                                                                              5546                                                                             220                                                                              130                                                                              5.46                                                                             5.16                                                                             0.94                                                                             220                                                                              276                                                                              0.41                                                                             0.44                                                                             0.92                                  64 1601                                                                             1537                                                                              6418                                                                             230                                                                              130                                                                              4.74                                                                             4.47                                                                             0.94                                                                             291                                                                              381                                                                              0.40                                                                             0.44                                                                             0.90                                  65 1919                                                                             1929                                                                              6343                                                                             230                                                                              160                                                                              3.92                                                                             3.53                                                                             0.90                                                                             334                                                                              426                                                                              0.38                                                                             0.42                                                                             0.91                                  66 2782                                                                             1343                                                                             12576                                                                             260                                                                              280                                                                              5.26                                                                             9.65                                                                             1.83                                                                             488                                                                              578                                                                              0.56                                                                             0.61                                                                             0.92                                  67 3939                                                                             1343                                                                             10009                                                                             260                                                                              200                                                                              2.89                                                                             7.44                                                                             2.58                                                                             534                                                                              599                                                                              0.61                                                                             0.66                                                                             0.93                                  68 3952                                                                             1925                                                                              9771                                                                             260                                                                              200                                                                              2.89                                                                             5.22                                                                             1.81                                                                             622                                                                              765                                                                              0.51                                                                             0.57                                                                             0.89                                  __________________________________________________________________________     .sup.1 Number reported in thousands                                      

As generally shown in Table II, the sand loadings of the air and fluegas, α_(a) and α_(g), were varied in the range of from about 2 to about20. An increase in the sand loading at a given fluid flow rate will tendto provide an increase in the percent heat recovery Hr and the pressuredrop, as previously discussed. In the uncoupled mode of operation, theair and flue gas flow rates can be independently varied and the testruns reported in Table II reflect variations in the weight ratio of theair to flue gas flow rates, β, ranging from about 1 to about 3. Thequantity of heat transferred will increase with increases in β and theeffects of such variations are conveniently illustrated by considerationof the sensitivities of the ideal air and flue gas outlet temperaturest_(a2) and t_(g2) to variations of the air and flue gas flow rates.Through the use of the heat balance and similar techniques as discussedabove, the following differential equations are obtained reflecting thesensitivities of the ideal air outlet temperature to the air flow rate aand the ideal flue gas outlet temperature to the flue gas flow rate g.##EQU6##

The sensitivities of the outlet temperatures, as shown by equations (5)and (6), similarly respond to variations in the air and flue gas flowrates, with t_(a2) decreasing with increases in a and t_(g2) increasingwith increases in g. The effect of varying β is illustrated byconsidering the responsiveness of the air outlet temperature to equalfractional changes in the air flow rate and in the sand flow rate sincein practical heat exchanger applications the sand and air flow rateswill be determined in view of the primary fluid or flue gas flow rate ofthe process to which the heat exchanger is applied. Accordingly,assuming the same percent weight changes in s and a or, ds/s=da/a, thefollowing equation is obtained for the air/sand sensitivity ratio.##EQU7##

The air outlet temperature is more sensitive, and is in the oppositedirection, to change in the air flow rate than to change in the sandflow rate as shown by equation (7). This relationship is illustrated inFIG. 13 where the air/sand sensitivity ratio for the indicated β valuesare shown as a function of α_(a). Assuming an α_(a) value of 2 and a βof 1, the air outlet temperature is three times more sensitive to achange in the air flow rate as compared with an identical weightpercentage change in the sand flow rate. If the α_(a) value is increasedto 4, indicating a higher sand loading, and β is maintained at a valueof 1, the air/sand sensitivity ratio increases to a value of 5. Thisfurther increase in the ratio to a value of 5 reflects that the airoutlet temperature is less responsive to variations in the sand flowrate in the upper range of sand flow rates as discussed above withrespect to FIG. 11. Similar observations may be made for the furthervalues of β as shown in FIG. 13. In all cases, modulation of the sandflow rate enables relatively more precise regulation of the outlettemperature t_(a2).

The percent heat recovery Hr is responsive to variations in β, and therelationship between Hr and β at variable sand flow rates is illustratedby consideration of the theoretical operation of heat exchanger 10 at afixed flue gas flow rate and constant flue gas and air inlettemperatures. Under these conditions, a theoretical percent heatrecovery Hr*, as defined above, may be determined for the heat exchanger10. For this purpose, the ideal air outlet temperature t_(a2) isdetermined under assumed operating conditions using equation (1), andthe resulting quantity of heat transferred is compared with the quantityheat theoretically available if the flue gas inlet temperature t_(g1)was reduced to the air inlet temperature t_(a1). This is expressedmathematically as follows: ##EQU8##

In equation (8), t_(a2) is determined by means of equation (1) asindicated above and substitution of equation (1) into equation (8)results in the following expression for Hr*: ##EQU9##

The theoretical percent heat recovery Hr* is shown as a function ofα_(a) for the indicated values of β in FIG. 14. The α_(a) value isselected in this instance since it reflects variations in the quantityof heat transferred resulting from the relative sand loading per se andit is indicative of the pressure drop of the heat exchanger resultingfrom the pneumatic conveyance of the sand through the lift line 20. Therelative sand loading of the flue gas, α_(g), is not affected byvariation of β at a fixed flue gas flow rate, but it is affected by achange in the sand flow rate and results in corresponding changes in thepressure drop and the quantity of heat transferred on the flue gas sideof the heat exchanger as the sand is pneumatically conveyed through thelift line 24. However, it is convenient in FIG. 14 to reference thepercent heat recovery against the α_(a) values for the indicated βvalues since the α_(a) value is determined by the selection of the α_(a)and β values in a system having a fixed flue gas flow rate as consideredherein and generally encountered in practical applications of the heatexchanger.

Referring to FIG. 14, as the sand loading or α_(a) value is increased ata given value of β, the percent heat recovery and air outlet temperaturewill each increase in the same manner as discussed above with respect toFIG. 11. Further, it is apparent from FIG. 14 that Hr* will increasewith increasing β values. An increase in the β value and, moreparticularly, the air flow rate herein enables higher sand flow rates tobe used without corresponding increases in pressure drop on the air sideof the heat exchanger. Accordingly, operation of the heat exchanger withunequal air and flue gas flow rates and β values greater than 1 isparticularly advantageous.

The Hr* value tends to approach a limit for each value of β as the sandflow rate increases. These limits can be determined from equation (9) bydeletion of the reciprocal α_(a) term as its value approaches zero atincreasing values of α_(a) resulting from increasing sand flow rates.Under these conditions, equation (9) indicates that for a β value of 1,the limit of Hr* is 50%. Similarly, the limit of Hr* for a β value of 2is 66.7% and, for a β value of 3, the limit of Hr* is 75%.

The responsiveness of the percent heat recovery to specific variationsin β at variable sand flow rates and the resulting fluid outlettemperatures are illustrated with respect to FIG. 14 through the use ofTable III below. The resulting ideal outlet temperatures t_(a2), t_(g2)and theoretical percent heat recovery Hr* were calculated for theindicated operating conditions for an assumed flue gas inlet temperaturet_(g1) equal to 2,000° F. and air inlet temperature t_(a1) equal to 100°F. The specific heats of the air, flue gas and sand were assumed to beequal throughout the indicated range of temperatures.

                  TABLE III                                                       ______________________________________                                        Point                                                                              a       g      s     α.sub.a                                                                     α.sub.g                                                                      β                                                                            t.sub.g2                                                                            t.sub.a2                                                                           Hr*                         ______________________________________                                        A    1000    1000   1000  1   1    1   1367  733  33.3                        B    "       "      2000  2   2    1   1240  860  40.0                        C    2000    "      1000  0.5 1    2   "     480  40.0                        D    "       "      2000  1   2    2   1050  575  50.0                        E    1500    "      3000  2   3    1.5 "     733  50.0                        F    2000    "      2667  1.3 2.7  2    985  605  53.4                        G    3000    "      3000  1   3    3    860  480  60.0                        ______________________________________                                    

Referring to Table III and FIG. 14, the steady state operating point Afor the heat exchanger 10 is shown. At operating point A, the air, fluegas and sand flow rates are each 1,000 lbs./hr. and the theoreticalpercent heat recovery Hr* is about 33%. Under these conditions, the fluegas temperature is reduced from 2,000° F. to 1,367° F. and the airtemperature is increased from 100° F. to 733° F.

Assuming that the sand flow rate is increased to 2,000 lbs./hr. and theheat exchanger is allowed to reach steady state, operation at point B isobtained. The increase in the sand flow rate results in an increase inthe α_(a) and α_(g) values and the value of β remains 1. Under theseconditions of operation at point B, the air outlet temperature isincreased to 860° F., the percent heat recovery Hr* is increased to 40%and the flue gas outlet temperature is decreased to 1,240° F. ascompared with point A operation.

If steady state operation is again assumed at point A and the air flowrate is doubled instead of the sand flow rate, operation at point C isattained. In this instance, α_(a) is decreased to 0.5, β is increased to2, and the percent heat recovery is again increased to 40%. Although anequal increase in the percent heat recovery is obtained upon equalincreases in the air or sand flow rates provided the latter areinitially equal, the air outlet temperature is reduced to 480° F. byoperation at point C as compared with point B and the same flue gasoutlet temperatures result.

If the steady state operation at point C is varied by increasing thesand flow rate to 2,000 lbs./hr., the sand loading or α_(a) valueincreases to 1 along the curve for β equal to 2, and operation at pointD is attained. As shown in Table III, the increase in the sand loadingas compared with point C results in an increase in t_(a2) to 570° F. andthe percent heat recovery is increased to 50%. Further increases in thesand loading to result in values of α_(a) equal to about 3.5 will resultin additional increases in the outlet temperature t_(a2) and about anadditional 11% increase in Hr* to a value of about 61%. As the sandloading is further increased to provide values of α_(a) on the order ofabout 5 and greater, the further increases in the outlet temperaturest_(a2) are not as great and the Hr* limit value of 66.7% is approachedfor operation at a β value of 2.0.

A comparison of operation at points A and D indicates that the totalheat recovery when both the air and sand flow rates are increased anequal amount is greater than the sum of the individual recoveriesprovided by an increase in either of the flow rates alone. Specifically,operation at points B and C each result in about a 6.7% increase in Hr*as compared with point A in contrast with the 16.7% increase in the Hr*value obtained by operation at point D. This synergistic effectcontinues as both the air and sand flow rates are further increased tothe design limits of the system and the synergistic effect, or, moreparticularly, the additional quantity of heat transferred relative tothat resulting from the individual changes, progressively decreasessince the total quantity of heat available for recovery is fixed.

In further illustration of the variable area operation of the heatexchanger, operation at point E is considered. Under the conditionsindicated in Table III, a 50% heat recovery is obtained and the airoutlet temperature is 733° F. Thus, as compared with operation at pointA, the same air outlet temperature is maintained and the percent heatrecovery is increased. It should be appreciated that the total heatexchanger pressure drop will be larger at point E as compared withpoints A or D. However, the flexibility of the variable area system iswell illustrated by the foregoing examples and recognition of the factthat both the air flow rate and the sand flow rate are continuouslyvariable throughout the designed range of operation of the heatexchanger.

In illustration of the effect of varying β while maintaining a constanttotal heat exchanger pressure drop, reference is made to operatingpoints B, F and G of Table III. The indicated air and sand flow ratesfor these operating points result in increasing β values and a constanttotal value for the sum of α_(a) plus α_(g), the latter being indicativeof a constant total pressure drop with recognition that the actualpressure drops displayed by the two sides of the exchanger vary asdiscussed above with respect to FIG. 10. As indicated in Table III,theoretical percent heat recovery values of 40%, 53% and 60% arerespectively obtained for β values of 1, 2 and 3. The increases in thepercent heat recovery are accompanied by decreases in the secondaryfluid outlet temperature, and t_(a2) values of 860° F., 605° F. and 480°F. are obtained. Generally, the β value will not be greater than about 5since the resulting temperature at which the secondary fluid isrecovered will be relatively low in the absence of unusual userrequirements. Accordingly, the heat exchanger may be designed andoperated to recover the flue gas heat in the secondary fluid at variousflow rates and temperatures depending upon user requirements.

Referring to FIG. 15, the relationship between the percent heat recoveryHr and the total heat exchanger lift line pressure drops is shown forselected test runs of Tables I and II. In this instance, the test runsare grouped in accordance with the indicated β ranges and a single curveis drawn for each β range for purposes of convenience. The curves ofFIG. 15 and the dotted portions thereof are drawn in accordance with therelationships shown in FIG. 14.

As shown in FIG. 15, the same relationships between Hr and β at variablesand flow rates are observed with respect to the total heat exchangerlift line pressure drops or α_(a) as discussed above with respect toFIG. 14, since the pressure drop is a function of both the sand flowrate and the absolute air and flue gas flow rates. Accordingly, thepercent heat recovery will increase at a given value of β as the totalsystem pressure drop increases as a result of increased sand flow rates.Similarly, increases in the percent heat recovery are obtained withincreasing values of β and such increases may be obtained at asubstantially constant total system pressure drop. For example, if β isvaried through the ranges indicated in FIG. 15 by increasing the airflow rate as well as the sand flow rate to maintain a total pressuredrop of 650 mm of water, the percent heat recovery is increased fromabout 43% to 69%. The additional heat recovered by increasing β isrecovered at a lower air outlet temperature assuming a constant flue gasinlet temperature and flow rate.

The actual percent heat recovery achieved closely follows thetheoretical recovery, as shown by the efficiency values, E, reported inTables I and II. The reported efficiencies effectively compare theactual percent heat recovery with the theoretical percent heat recoverywhich could be expected if ideal cocurrent heat exchange was attained.As shown in Tables I and II, the efficiency values are generally 90% ormore without allowance for heat losses. The efficiency of the heatexchanger in transferring heat between the primary and secondary fluidsis substantially unaffected by the specific operating conditions such asthe flue gas inlet temperature, the absolute values of the air and gasflow rates and the total heat flux. Accordingly, the design andoperation of the exchanger is not restricted per se to optimum operatingefficiency ranges or conditions, but rather, the exchanger is readilyapplied to a wide variety of systems and displays the same relativelyhigh efficiencies for the selected level of heat recovery at apredetermined pressure drop and/or a desired fluid outlet temperature.

The foregoing characteristics of the heat exchanger system 10 enable itto be operated as a variable area heat exchanger with acceptable levelsof heat recovery upon modulation of the sand flow rate and area of theheat exchange surface as a function of a preselected pressure dropand/or outlet temperature of the exchanger itself or of the system towhich the exchanger is applied. Variable area operation is mosteffective at relatively lower pressure drops, α_(a) values no greaterthan about 5 and generally in the range of from 1 to 4, where thesecondary fluid outlet temperature and the relative amount of heattransferred display greater sensitivity to changes in the sand flow rateor area of the heat exchange surface. Further, operation at relativelylower α_(a) values is also favored since the quantity of heattransferred and effect upon the outlet temperature per unit of sand floware not as costly in terms of pressure drop. The achievement of violentmixing zones as a characteristic of the pneumatic conveyance is readilyobtained for the indicated values and contributes to the efficiency ofheat transfer, as previously discussed.

The variable area capabilities of the heat exchanger enable it to bereadily integrated with a system to which it is applied. For example, ifthe exchanger 10 is applied to a process which itself has a variablepressure drop, the control arrangement 54 may be arranged to sense thecombined pressure drop of the system 10 and such other process withmodulation of the sand flow rate to achieve a desired combined pressuredrop or to deliver the secondary fluid to the other process at a desiredpressure. Further, an operating temperature of the applied system couldbe monitored as a control parameter in the feed control arrangement 54and combined with pressure drop regulation to assure a maximum sand flowrate which is less than the choking velocity of the heat exchanger liftlines. In all cases, the operation of the heat exchanger is controlledwith modulation of the sand flow rate to provide direct internalvariation of the area of the heat exchange surface and the desiredpressure drop and/or fluid outlet temperature.

It is apparent that the heat exchanger 10 may be efficiently andeffectively employed with preselected operating characteristics toaccommodate the requirements of the system to which it is applied. Ifmaximization of the overall quantity of heat recovered is desired, theexchanger 10 may be operated at relatively high sand flow rates andpressure drops or a cascade arrangement comprising two heat exchangers10 connected in series can be used to obtain a substantially maximumpercent heat recovery with a minimization of the power requirement andexpense since the total power requirement for the cascaded system wouldstill represent a reduction in cost. Alternatively, the relatively coolflue gas after traveling through the heat exchanger 10 can be directlyhandled in a more conventional heat exchanger such as a shell and tubearrangement.

In illustration of cocurrent heat transfer in accordance with thepresent invention, intermediate temperatures of the air and flue gasduring pneumatic conveyance are reported for the test runs of Table IV.In these runs, the heat exchanger 10 was modified to substitute settlingchambers for the vessels 12,14, the bonnets 40, 44 being retained, andthe outlet temperatures t_(a2) and t_(g2) were measured at the outletsof the settling chambers. The phase separation of the sand and thefluids was substantially achieved through the use of the bonnets 40,44.

                                      TABLE IV                                    __________________________________________________________________________    Run No.                                                                            g  a  s  t.sub.s1                                                                         t.sub.a1                                                                         t.sub.a4                                                                         t.sub.a5                                                                         t.sub.a2                                                                         t.sub.s2                                                                         t.sub.g1                                                                         t.sub.g6                                                                         t.sub.g7                                                                         t.sub.g2                                                                         E                                 __________________________________________________________________________    69   1146                                                                             1529                                                                             5633                                                                              990                                                                             125                                                                              720                                                                              800                                                                              800                                                                              830                                                                              1785                                                                             1020                                                                              990                                                                              930                                                                             1.00                              70   1529                                                                             1529                                                                             5568                                                                             1065                                                                             130                                                                              720                                                                              850                                                                              850                                                                              890                                                                              1790                                                                             1220                                                                             1100                                                                             1040                                                                              .93                              71   1911                                                                             1911                                                                             5904                                                                             1020                                                                             130                                                                              650                                                                              750                                                                              810                                                                              830                                                                              1650                                                                             1200                                                                             1080                                                                             1020                                                                              .98                              72   1911                                                                             3057                                                                             5200                                                                              960                                                                             130                                                                              390                                                                              620                                                                              665                                                                              700                                                                              1650                                                                             1140                                                                              990                                                                              960                                                                             1.04                              __________________________________________________________________________

As shown by the runs reported in Table IV, the bulk of the heat transferis completed in the first part of each of the lift lines for typicalsand, air and flue gas flow rates and efficient operation of the heatexchanger 10 is achieved in all cases. The rapidity of the heat transferprocess is particularly illustrated by run Nos. 69 and 70, and it isdemonstrated by comparison of the fluid temperatures at the indicatedpoints during pneumatic conveyance in each of the lift lines with theoutlet temperatures t_(a2), t_(g2). The temperature profile for each ofthe lift lines is shown in FIG. 16 for run No. 69 wherein it is apparentthat the outlet temperatures t_(a2), t_(g2) are substantially obtainedat the two-foot level and fully developed at the seven-foot level. Underthese operating conditions, the heat transfer process is substantiallycompleted by the time the gas-solids streams exit from the lift lines.

The rapidity of the heat transfer process generally decreases withincreasing air and flue gas flow rates, as shown by run Nos. 71 and 72in Table IV. Under such operating conditions, reasonable quantities ofheat continue to be transferred throughout the lift line portion of thepneumatic conveying step, and the further heat transfer which occurs asa result of the violent mixing within the bonnets 40,44 becomesapparent. This is illustrated in FIG. 16 for run No. 71. As shown inFIG. 16, a substantial portion of the heat transfer again occurs priorto the time the fluids reach the two-foot level, and a moderate degreeof heat transfer is experienced as the fluids continue to flow throughthe lift lines.

The quantity of heat transferred by the time the fluids exit from thelift lines as indicated by temperatures t_(a5), t_(a7), is believed tobe affected to a degree by the further heat transfer occurring withinthe bonnets 40,44, since these temperatures are sensed at a locationabout 18 inches below the violent mixing occurring within the bonnets.However, the further heat transfer indicated by the dotted portions ofthe curves in FIG. 16 for Run No. 71 is believed to be exclusivelyassociated with the bonnets, since the temperatures t_(a2),t_(g2) areeach sensed about one foot downstream of the bonnets and the sand issubstantially disengaged from the fluid phases within the bonnets.

Referring to FIGS. 17 through 19, a heat exchanger 100 of modifieddesign is shown. The heat exchanger 100 includes first and secondoperating or separation vessels 102 and 104 which comprise adjacent,axially aligned, horizontal cyclones separated by a common wall 105. Asolid particulate material 106, similar to the sand 16, is circulatedbetween the vessels 102, 104 for purposes of heat transfer with gaseousmedia. The particulate material 106 is withdrawn from the vessel 102 bymeans of a downcomer 108 and pneumatically transferred to the vessel 104through an associated lift line 110. Similarly, the vessel 104 isprovided with a downcomer 112 communicating with an associated lift line114 for purposes of transporting the particulate material 106 from thevessel 104 to the vessel 102.

In this embodiment, the primary fluid or hot flue gas from which heat isto be recovered is introduced into the lift line 114 through a lowerlift line elbow 116, and the cooled flue gas is vented through a cycloneoutlet 118. On the other side of the heat exchanger 100, the secondaryfluid or ambient air is introduced into the lift line 110 through alower lift line elbow 120 which is identical in structure with the elbow116, and the preheated air is removed from the vessel 104 through acyclone outlet 122.

The downcomers 108, 112 respectively include horizontal leg portions108a and 112a. Accordingly, the particulate material 106 is withdrawnthrough each of the downcomers 108, 112 as a loose packed bed column anda gravity feed-lock technique identical to that employed in theembodiment of FIG. 1 is used. To that end, each of the downcomers isprovided with a biasing means or trigger air supply to cause theparticulate material 106 accumulated in the leg portions 108a and 112ato spill over into the respective lift lines 110 and 114 duringautomatic feed. Further, it is also convenient to employ radial feed ofparticulate material into the lift line 110 and tangential feed of theparticulate material into the lift line 114 for the same reasons asindicated with respect to the embodiment of FIG. 1.

The heat exchanger 100 includes a feeder vessel 124 for providing anautomatic make-up supply of particulate material 106. The feeder 124 hasa supply leg 126 extending below the level of the desired inventory ofparticulate material retained above the downcomer 106. A pressure line128 communicates between the feeder 124 and the vessel 102 for purposesof maintaining equal gas pressures in each of the vessels. The supplyleg 126 includes an outlet opening 126a located at the desired level ofinventory of particulate material 106 above the downcomer 108. If thelevel of the inventory falls below the desired value, additionalparticulate material 106 will flow through the supply leg 126 until anaccumulation of particulate material is formed at its natural angle ofrepose adjacent the outlet opening 126a.

With particular reference to FIGS. 18 and 19, the details of the vessel102, the downcomer 108, and the lift line 114 are shown, it beingunderstood that the vessel 104 and its downcomer and lift line aresimilarly constructed. The particulate material 106 being pneumaticallyconveyed through the lift line 114, while undergoing cocurrent heattransfer with the gaseous conveying medium, enters into the vessel 102through an inlet opening 102a. The particulate material 106 sweepsaround the inside periphery of the vessel 102 in a counterclockwisedirection, as shown in FIG. 19. As the velocity of the particulatematerial 106 decreases and separation from the gaseous conveying mediaoccurs, the particulate material passes through a vessel outlet opening102b disposed at an angularly remote location relative to the inletopening 102a. The outlet 102b opens into a downcomer collection trough108b, which in turn communicates with the downcomer 108 and receives theinventory of particulate material 106. The collection trough 108b isconnected about the opening 102b into the vessel 102 with a fluid tightseal to prevent the escape of gaseous media or particulate material.

The heat exchanger 100 is controlled in the same manner as the heatexchanger 10. Accordingly, the automatic control arrangements 54 and 70are incorporated in the heat exchanger 100 in the same manner asemployed in the heat exchanger system 10. More particularly, the feedcontrol arrangement 54 is arranged to regulate the withdrawal ofparticulate material 106 through the downcomer 108 from the vessel 102as a function of the pressure drop through the lift line 110. Thecontrol arrangement 70 is again employed in a following manner toregulate the level of inventory in a downcomer collection trough 112b bymodulation of the trigger air supply to the leg portion 112a. Further,the temperature sensor 63 and temperature control arrangement areemployed in the heat exchanger 100 by sensing the outlet temperature ofthe secondary fluid or air exiting from the cyclone outlet.

The heat exchanger 100 is particularly structurally efficient, since thevessels 102, 104 comprise cyclonic separators, and the unit is designedto illustrate the minimum inventory requirement for the particulatematerial 106b. In practice, a 500,000 Btu/hr. unit requires only 6square feet of floor space and a 10-foot vertical clearance. Asindicated above, the heat exchanger 100 is comparable in capacity andcapability with the heat exchanger system 10 and, accordingly, the heatexchanger 100 also provides similar advantages over prior art devices.

Referring to FIG. 20, a heat exchanger system or unit 200 having a zeroloop flow path and arranged to recover heat from hot flue gas is shown.The heat exchanger 200 includes a first operating vessel 202 and asecond operating vessel 204 between which particulate material 206 iscirculated for purposes of heat transfer with gaseous media.Accordingly, the particulate material is withdrawn from a flowinginventory thereof maintained in the lower portion of the vessel 202through a downcomer 208 and pneumatically transferred to the vessel 204through an associated lift line 210. The particulate material 206 isseparated from the gaseous conveying medium in the vessel 204 andcollected within a flowing inventory thereof within the vessel. Theparticulate material 206 is withdrawn from the vessel 204 through anassociated downcomer 212 and transferred to the vessel 202 forcountercurrent contact with the gaseous stream flowing upwardly throughthe vessel.

For purposes of illustration, the heat exchanger 200 is shown applied toa burner-furnace simulator 214 which is substantially identical to thesimulator 34 and generates a primary fluid comprising hot flue gascontaining sufficient thermal energy to warrant heat recovery. The hotflue gas generated by the simulator 214 is introduced through flue gasinlet 216 into the vessel 202. The flue gas flows upwardly through thevessel 202 in countercurrent contact with the downwardly movingparticulate material 206, as described in greater detail below, and thecooled flue gas is withdrawn from the vessel 202 through an upper fluegas outlet 218. Typically, the flow of flue gas through the vessel 202will be sustained by natural chimney draft, and countercurrent heatexchange and phase separation are achieved with no significant pressuredrop or energy requirement on this side of the heat exchanger. However,a suction fan (not shown) may be applied to the vessel 202 if such isrequired in a particular application. The secondary fluid to which theheat recovered from the flue gas is to be transferred during thepneumatic conveyance of the particulate material comprises a stream ofambient air delivered to the lift line 210 by a blower 220. Thesecondary fluid may be used as preheated combustion air in the processfrom which the flue gas is derived, or it may be used in a separateprocess.

The vessel 202 has a generally cylindrical configuration, with taperedend portions and an intermediate baffle assembly 222 which is arrangedto assure effective countercurrent heat exchange contact between theflue gas and downwardly flowing particulate material 206 while defininga sufficiently open gaseous flow path to assure a natural draft forpurposes of maintaining flue gas flow. The baffle assembly 222 includesopen-ended funnel baffles 224 and intermediately disposed conedistributor baffles 226. The upper open end of each of the baffles 224has a diameter substantially corresponding with the inside diameter ofthe cylindrical portion of the vessel 202, and the upper ends of each ofthe baffles 224 are secured to the adjacent wall of the vessel 202, asby welding. The upper ends of the baffles 226 are sized to receive thelift line 210, and they are secured to the lift line by any convenientmeans, such as welding.

In FIG. 20, the downward flow of the particulate material 206 throughthe baffle assembly 222 is somewhat diagrammatically shown for purposesof clarity of illustration, and it should be appreciated that theparticulate material 206 actually flows downwardly through the baffleassembly 222 as an annular curtain. To that end, the downcomer 212encircles an upper portion of the lift line 210, and an annular shaped,loose packed bed column of particulate material is formed between theadjacent concentric walls of the downcomer and the lift line. Thedowncomer 212 includes a laterally extending portion 212a mounted to thelift line 210. The laterally extending portion 212a includes an upperannular surface 212b which is spaced below the lower end of the radiallyconfining wall of the downcomer 212 to provide an accumulation ofparticulate material adjacent the bottom of the column thereof containedwithin the downcomer. Upon application of trigger air, the particulatematerial spills from the accumulation into the upwardly flowing flue gasin an annular flow pattern.

The particulate material spilling from the accumulation thereof adjacentthe bottom of the downcomer 212 is guided by a cone surface 212c of theportion 212a into a radially outward flow direction for engagement withthe adjacent baffle 224. As shown in FIG. 20, the radially outward flowdirection is reversed upon engagement with the baffle 224, and theparticulate material is then directed in a radially inward flowdirection into engagement with the adjacent baffle 226. The radiallyinward flow direction is again reversed by the baffle 226, and theparticulate material is then directed into engagement with the adjacentlower baffle 224. This flow pattern of the particulate material isrepeated as it passes downwardly into engagement with the remainingbaffles 224 and 226 as a cascading annular curtain of particles.

The flue gas entering the vessel 202 through the inlet 216 passes in atransverse direction through the adjacent annular portion of the curtainof particulate material falling from the lowermost baffle 224 and theremote portion of the annular curtain flows along a truncating end wall202a of the vessel 202 and serves to insulate the vessel wall from theincoming flow of hot flue gas. The flue gas flows upwardly and traverseseach of the curtains of particulate material formed between the baffles222 and 224 as shown by the flow arrows. Upon traversing the curtain ofparticulate material formed between the laterally extending portion 212aand the adjacent baffle 224, the now cooled flue gas passes upwardlythrough the annular space between the vessels 202 and 204 and it isvented through the outlet 218. In this manner, countercurrent heatexchange is provided on the flue gas side of the heat exchanger 200, andthe baffle assembly 222 effectively provides an eight-passcountercurrent flow exchanger.

Following countercurrent heat exchange with the flue gas, the hotparticulate material 206 is directed by the wall 202a to the open upperend of the downcomer 208 and withdrawn from the bottom of the vessel 202through the downcomer. The loose packed bed column of particulatematerial formed in the vertical portion of the downcomer 208 provides adynamic seal between the flue gas and the air to be heated in the liftline 210. An accumulation of particulate material is formed in thelaterally extending portion 208a of the downcomer 208, and trigger airis used to bias the particulate material into the lift line 210.

In the lift line 210, the particulate material undergoes cocurrent heatexchange with the ambient air as the latter pneumatically conveys itinto the vessel 204. Upon exiting from the lift line 210, the gas-solidsstream is impacted against a bonnet 228 for purposes of phase separationand further heat transfer in a manner similar to that discussed in theprior embodiments. The relatively cool particulate material is collectedwithin the vessel 204 and withdrawn therefrom through the downcomer 212.The heated ambient air exits from the vessel 204 through an outlet 230.Accordingly, the heat transfer process on the secondary fluid side ofthe heat exchanger 200 is substantially identical with that described inprevious embodiments.

The location of the lift line 210 and vessel 204 within the vessel 202is structurally compact and minimizes heat loss on the air side of theexchanger 200. However, it should be appreciated that the lift line 210and vessel 204 may be arranged exteriorly of the vessel 202. Further, inan exterior arrangement or in the illustrated arrangement, it is notnecessary that the downcomer 212 concentrically receive the upperportion of the lift line 210, since the downcomer and lift line may belaterally spaced as in the heat exchangers 10 and 100. In sucharrangements, the flow of particulate material from the downcomer 212may be distributed into the flue gas and baffle assembly 222 in asuitable flow pattern through the use of an initial baffle 226 or afunctional equivalent thereof. In all cases, the column of particulatematerial within the downcomer 212 forms a dynamic seal and anaccumulation of particulate material adjacent the bottom of the columnis used for controlling the introduction of the particulate materialinto the stream of flue gas.

The dynamic seal provided within the downcomer 212 essentially preventsthe flow of air, which is at a positive pressure, to the flue gas sideof the system, which is at a negative chimney draft pressure. If theseal in the downcomer 212 fails for any reason or if an irregular orinterrupted flow of particulate material occurs on the flue gas side,the flue gas continues to vent by virtue of the natural draft withoutsubstantial effect upon the process to which the heat exchanger isapplied. Of course, this tends to mitigate the requirements of thecontrol process and the control apparatus for the heat exchanger 200.

The heat exchanger 200 may be controlled using devices and techniquessimilar to those used in connection with the heat exchanger 10.Accordingly, the automatic control arrangements 54 and 70 may be used torespectively control the flow of particulate material through thedowncomers 208 and 212. To that end, trigger air is supplied through aline 232 to bias the particulate material into the lift line 210 and, inthe case of the downcomer 212, it is convenient to supply trigger airthrough a line 234, which is subsequently divided to provide a number ofangularly spaced jets of biasing trigger air adjacent the accumulationof particulate material at the bottom of the downcomer 212.

For purposes of recovering heat energy by means of heat transfer betweenprimary and secondary fluids, the heat exchanger 200 is cocurrent inthermal performance even though countercurrent heat transfer is employedon the flue gas side of the system. This is true since cocurrent heattransfer is employed on the air side of the system and the lowesttemperature to which the flue gas can be cooled is equal to thetemperature of the particulate material introduced into the flue gas viathe downcomer 212, which temperature is, in turn, equal to the outletair temperature. The total significance of countercurrent heat transferon one side of the system has not yet been fully evaluated and theoperation of the zero loop system is not directly described by the abovemathematical relationships for the figure 8 flow path system. However,the heat exchanger 200 has demonstrated controllable steady stateoperating characteristics and efficient heat recovery between primaryand secondary fluids or gaseous streams.

The heat exchanger 200 was operated using a particulate materialcomprising tabular alumina having an 800 micron particle size. Theparticulate material was selected to assure its downward flow throughthe flue gas under the anticipated flue gas flow conditions in order toeffect counter-current heat transfer and phase separation. The rate atwhich the particulate material falls through the flue gas is to a degreea function of the specific particulate material employed, and variationsin the rate will tend to result in corresponding variations in theresidence time and heat transfer. Generally, a wide range of particulatematerials reflecting the considerations outlined above with respect tothe figure 8 system will be useful in the zero loop system, which tendsto impose a somewhat lesser physical working burden on the particulatematerial.

In the operation of the exchanger 200, the flow rate of the alumina wasmaintained in this instance by setting a constant biasing trigger airflow through line 232. The level of alumina within vessel 204 wasregulated with a level control arrangement as described above and theuse of a biasing trigger air flow through the line 234. The heatexchanger was operated at conditions varying over its designed range ofoperation and approaching its designed capacity of 500,000 BTU/hr., andat inlet flue gas temperatures as high as 2400° F. In all cases, stablesteady state operation was obtained with α_(a) and α_(g) values eachranging from about 0.5 to about 15 and β values varying from about 1.0to about 2.5. The percent heat recovery ranged from about 35% to about75%.

The countercurrent flow occurring on the flue gas side of the heatexchanger within the vessel 202 is characterized by a relatively lowervelocity difference between the alumina particles and flue gas, as wellas less turbulent flow conditions as compared with the air side of theexchanger and, accordingly, the coefficient of heat transfer on the fluegas side is less than that experienced on the air side. In view of thesame, it is advantageous to operate at relatively high flow rates ofparticulate material and α_(g) values greater than 3. Thus, operation ofthe exchanger 200 at relatively high flow rates of particulate materialand corresponding α_(g) values as high as 15 is desirable and quitepractical, since no pressure drop penalty is experienced on the flue gasside of the exchanger. Of course, the high particulate material flowrates will also be experienced on the air side of the exchanger and theyare associated with relatively higher pressure drops in the lift line210. However, the overall pressure drop in the zero loop flow pathsystem of the exchanger 200 will be less than that to be expected in thefigure 8 flow path system of the exchanger 10, since the formereliminates the pressure drop associated with the violent mixing andconveyance of the particulate material during the heat exchange processon the flue gas side. Moreover, it is more economical to experiencepressure drop on the air side of the system, since the secondary fluidflow is provided by less expensive low temperature equipment as comparedwith the significantly more expensive high temperature blowers orsuction fans which would be required on the flue gas side of theexchanger.

The heat exchanger 200 is operable at variable flow rates of particulatematerial, and it has displayed variable area operation characteristicssimilar to those of the figure 8 system. In the heat exchanger 200,these characteristics are particularly displayed at relatively low α_(g)values, such as 3.0 or less.

The percent heat recovery is a function of the ratio of the air to fluegas flow rates or β value. The percent heat recovery increases withincreasing β values, and the actual percent heat recovery achievedclosely follows the theoretical recovery.

As demonstrated by the foregoing detailed discussion of the illustratedembodiments, gas-solids transport techniques in accordance with thepresent invention enable the provision of efficient and economical heatexchanger systems. To a large degree, the improvements of the presentinvention are a result of the simplicity of operation and functionaldirectness of the figure 8 and zero loop flow path techniques asrespectively shown in the schematic flow diagrams of FIGS. 21 and 22with reference to the elements of the heat exchangers 10 and 200.

Referring to FIG. 21, the figure 8 particulate material or solids flowpath is shown by a solid line. The flue gas (g) and the air (a) streamsas shown by the dotted lines are established at spaced locations in thefigure 8 flow path through the use of the operating chambers 12, 24 and14, 20. The operating chamber 12, 24 functionally corresponds with thevessel 12 and lift line 24, and the operating chamber 14, 20functionally corresponds with the vessel 14 and lift line 20. As shownin FIG. 21, the circulating solids are passed in direct cocurrentcontact with the flue gas and the air in the respective operatingchambers, and the flue gas and air streams are maintained separatethrough the use of the dynamic seals 22 and 18 formed by the solids inthe figure 8 flow path. As specifically illustrated, it is advantageousto use the dynamic seals 22 and 18 for purposes of directly introducingthe solids into the flue gas and air streams.

In FIG. 22, the zero loop solids flow path is again shown by a solidline and the flue gas (g) and air (a) streams are shown by the dottedlines. The flue gas stream is maintained and brought into countercurrentcontact with the solids through the use of the operating chamber 202shown at the left in FIG. 22, which corresponds with the vessel 202 inthe heat exchanger 200. The air stream is brought into cocurrent contactwith the solids during the pneumatic conveyance thereof through the useof the operating chamber 204, 210, which functionally corresponds withthe vessel 204 and lift line 210 in the heat exchanger 200. The flue gasand air streams are maintained separate by means of dynamic seals 208and 212 provided by the solids as they are circulated along the zeroloop flow path. As indicated with respect to the figure 8 loop, thedynamic seals are used to directly introduce the solids into contactwith the flue gas and air streams, as well as to maintain the separationof the streams.

The continuous solids flow path and in-line solids seals and feeds inboth the figure 8 and zero loop flow path systems, as well as the use ofpneumatic conveyance when cocurrent contact is desired, minimizetransport functional and structural requirements. Accordingly, thesystems are characterized by minimal operational requirements and theyare readily integrated into other processes.

What is claimed is:
 1. A method of heat exchange between gaseous mediaand a particulate material which provides a heat exchange surface as itis circulated between first and second vessels for direct heat transferwith gaseous media comprising respectively withdrawing said particulatematerial from said vessels through first and second downcomers andtransferring said particulate material to associated lift lines,pneumatically conveying said particulate material to said first vesselwith a first gaseous stream flowing through one of said lift lines andto said second vessel with a second gaseous stream flowing through theother of said lift lines, each of said streams undergoing a pressuredrop during the pneumatic conveyance of said particulate material,cocurrently transferring heat between said particulate material andgaseous stream in each of said lift lines, separating said particulatematerial from each of said gaseous streams and respectively collectingsaid particulate material in said vessels, and transferring heat betweensaid gaseous streams as a function of the flow rate weight ratios ofsaid particulate material to said gaseous stream in each of said liftlines and the flow rate weight ratio of said gaseous streams.
 2. Amethod as set forth in claim 1, wherein the flow rate of saidparticulate material is controlled as a function of the pressure drop ofat least one of said gaseous streams in one of said lift lines.
 3. Amethod as set forth in claim 1, wherein said pressure drop of said onegaseous stream is directly related to the flow rate of said particulatematerial in said one gaseous stream and inversely related to the flowrate of said one gaseous stream.
 4. A method as set forth in claim 1,wherein said first gaseous stream is a heat providing fluid and saidsecond gaseous stream is a heat receiving fluid, said flow rate weightratio of particulate material to each of said streams is in the range offrom about 1 to about 20, and said flow rate weight ratio of said secondgaseous stream to said first gaseous stream is in the range of fromabout 1.0 to about 5.0.
 5. A method as set forth in claim 1, wherein thestep of withdrawing particulate material through said downcomersincludes forming a loose packed bed column of particulate material ineach of said downcomers and controlling the bulk density of saidparticulate material in each of said downcomers to provide a dynamicseal preventing net leakage flow of gaseous media through the downcomersunder the influence of any pressure difference between the gaseous mediaat opposite ends of the downcomer.
 6. A method as set forth in claim 1,wherein the step of pneumatically conveying said particulate materialincludes violently mixing said particulate material and gaseous streamin at least one localized zone to provide a maximized quantity of heattransfer.
 7. A method of cocurrent heat exchange for direct heattransfer between gaseous media and particulate material which provides aheat exchange surface as it is circulated between first and secondoperating vessels comprising withdrawing particulate material from saidvessels through first and second downcomers, forming a loose packed bedcolumn of particulate material under the influence of gravity in each ofsaid downcomers, forming an accumulation of particulate materialadjacent the bottom of each of said columns to restrict the withdrawalof particulate material through said downcomers, biasing particulatematerial from said accumulation thereof adjacent the bottom of each ofsaid columns into an associated lift line, pneumatically conveying saidparticulate material to said first vessel with a first gaseous streamflowing through one of said lift lines and to said second vessel with asecond gaseous stream flowing through the other of said lift lines,cocurrently transferring heat between said particulate material and saidgaseous stream in each of said lift lines and substantially completingthe transfer of heat between said gaseous streams and particulatematerial during pneumatic conveyance, and separating said particulatematerial from each of said gaseous streams.
 8. A method as set forth inclaim 7, wherein the step of forming a loose packed bed column ofparticulate material in each of said downcomers includes controlling thebulk density of the particulate material in each of said downcomers toprovide a dynamic seal preventing net leakage flow of gaseous mediathrough said downcomers under the influence of any pressure differencebetween gaseous media at opposite ends of said downcomer.
 9. A method asset forth in claim 8, wherein the step of controlling the bulk densityof the particulate material includes providing at least one of saidcolumns with sufficient height to develop a pressure head under theinfluence of gravity at least equal to any pressure difference betweenthe gas media at opposite ends of said one column.
 10. A method as setforth in claim 7, wherein the step of forming said accumulationsincludes sloping the accumulated particulate material in its naturalangle of repose downwardly toward said associated lift line, and thestep of biasing said particulate material includes aerating theaccumulated particulate material to cause it to directly spill over intosaid associate lift line.
 11. A method as set forth in claim 10, whereinthe steps of forming said accumulations of particulate material andbiasing the accumulated particulate material provide the sole valvingfunction for the withdrawal of particulate material through saiddowncomers.
 12. A method as set forth in claim 7, wherein at least oneof said associated lift lines has a substantially cylindricalconfiguration, and the step of biasing the particulate material includestangentially flowing the particulate material into said lift line toform a layer of particulate material about the inside periphery of saidlift line.
 13. A method as set forth in claim 7, wherein the step ofseparating said particulate material and gaseous streams includescollecting the separated particulate material in an inventory thereof ineach of said vessels.
 14. A method as set forth in claim 13, includingthe step of adding make-up particulate material to the inventory ofparticulate material in one of said vessels as a function of the levelof said inventory in said one vessel.
 15. A method of heat exchangebetween gaseous media and particulate material which provides a heatexchange surface as it is circulated between first and second operatingvessels for direct heat exchange with gaseous media in a systemcharacterized by heat exchange operating variables including gaseousmedia outlet temperatures and pressure drops comprising respectivelywithdrawing particulate material from said vessels through first andsecond downcomers and transferring the particulate material toassociated lift lines, pneumatically conveying the particulate materialto said first vessel with a first gaseous stream flowing through one ofsaid lift lines and to said second vessel with a second gaseous streamflowing through the other of said lift lines, cocurrently transferringheat between said particulate material and said gaseous stream in eachof said lift lines, separating said particulate material from each ofsaid gaseous streams, sensing a heat exchange operating variable of saidsystem, and varying the area of said heat exchange surface as a functionof said sensed variable by controlling the rate of withdrawal ofparticulate material through at least one of said downcomers to recoverat least one of said gaseous streams at a preselected value of saidsensed operating variable.
 16. A method as set forth in claim 15,wherein the step of controlling the rate of withdrawal of particulatematerial includes regulation of the pressure drop of the gaseous streamconveying said particulate material through said lift line associatedwith said one downcomer.
 17. A method as set forth in claim 15, whereinsaid sensed heat exchange operating variable is the pressure drop ofsaid gaseous stream conveying said particulate material through saidlift line associated with said one downcomer.
 18. A method as set forthin claim 15, wherein said sensed heat exchange operating variable is theoutlet temperature of said one gaseous stream.
 19. A method as set forthin claim 15, wherein the step of varying the area of said heat exchangesurface includes controlling the rate of withdrawal of particulatematerial to provide a flow rate weight ratio of said particulatematerial to gaseous stream in each of said lift lines in the range offrom about 1 to about
 5. 20. A method of recovering heat from processgases comprising circulating a particulate material between first andsecond operating vessels in a figure 8 flow pattern, respectivelywithdrawing particulate material from said vessels through first andsecond downcomers and transferring the withdrawn particulate material toassociated lift lines for pneumatic conveyance, forming a stream of aprimary fluid comprising a process gas from which heat is to berecovered in one of said lift lines and a stream of a secondary fluidcomprising a gaseous medium to which the recovered heat is to betransferred in the other of said lift lines, pneumatically conveyingsaid particulate material through the respective lift lines to saidvessels by means of said fluid streams, cocurrently transferring heatdirectly between said particulate material and fluid streams duringpneumatic conveyance, separating said particulate material from saidfluid streams to respectively collect said particulate material in saidoperating vessels, recovering said secondary fluid at a preselectedelevated outlet temperature and/or pressure, and controlling the flow ofsaid particulate material through at least one of said downcomers as afunction of said preselected outlet temperature and/or pressure of therecovered secondary fluid.
 21. A method as set forth in claim 20,wherein said process gas is flue gas at a temperature in excess of 750°F.
 22. A method as set forth in claims 20 or 21, wherein the step ofrecovering said secondary fluid includes using the recovered secondaryfluid as preheated combustion air in a combustion process from whichsaid process gas is derived.
 23. A method of heat exchange betweenseparate streams of gaseous media using particulate material which iscirculated along a continuous flow path for separate contact at spacedlocations along the flow path with first and second gaseous streamscomprising, introducing said particulate material into said firstgaseous stream at a first location in said flow path for direct contactand heat transfer with the first gaseous stream as the particulatematerial is circulated along the flow path to a second location therein,separating said particulate material from said first gaseous stream atsaid second location and introducing said particulate material into saidsecond gaseous stream for direct contact and heat transfer with saidsecond gaseous stream as the particulate material is circulated alongthe flow path to said first location, separating said particulatematerial from said second gaseous stream at said first location andintroducing the particulate material into said first gaseous stream,monitoring the pressure drop of at least one of said gaseous streams andintroducing said particulate material into said at least one gaseousstream as a function of said pressure drop, and cocurrently exchangingheat between said particulate material and each of said gaseous streamsduring contact therewith.
 24. A method as set forth in claim 23,including substantially continuously circulating said particulate alongsaid flow path.
 25. A method as set forth in claims 23 or 24, includingcirculating said particulate material along said flow path substantiallysolely by the use of pneumatic conveyance and the force of gravity. 26.A method as set forth in claim 25, wherein said particulate material ispneumatically conveyed along said flow path by each of said gaseousstreams during cocurrent heat exchange therewith.
 27. A method as setforth in claim 25, including introducing said particulate material intosaid first and second gaseous streams through substantially structurallyunobstructed flow conduits located in said flow path at said first andsecond locations, and forming a column of particulate material in eachof said flow conduits having a sufficient bulk density to prevent thenet leakage flow therethrough of said gaseous streams.
 28. A method asset forth in claim 25, including introducing said particulate materialinto at least one of said first and second gaseous streams through asubstantially structurally unobstructed flow conduit located in saidflow path, said flow conduit extending in a substantially verticaldirection and terminating at a lower transversely extending portion, andmaintaining in said flow conduit a column of particulate material havinga sufficient height to develop a pressure head under the influence ofgravity at least equal to any pressure difference between the gas mediaat opposite ends of said column.
 29. a method as set forth in claim 23,including violently mixing said particulate material and each of saidgaseous streams respectively in at least one localized zone duringcocurrent heat exchange therewith to maximize the quantity of heattransfer.
 30. A method as set forth in claim 23 wherein said pressuredrop is inversely proportional to the weight flow rate of said at leastone gaseous stream.
 31. A method as set forth in claims 29 or 30,wherein the other of said gaseous streams comprises a hot flue gashaving a temperature in excess of 750° F.
 32. A method as set forth inclaim 30, wherein the other of said gaseous streams comprises hotprocess gas.
 33. A method as set forth in claim 30, wherein the other ofsaid gaseous streams is a heat providing primary fluid and said at leastone of said gaseous streams is a heat receiving secondary fluid, andsaid particulate material is circulated at a flow rate weight ratio tosaid primary fluid in the range of from about 3 to about
 15. 34. Amethod as set forth in claim 23, wherein said first gaseous stream is aheat providing primary fluid and said second gaseous stream is a heatreceiving secondary fluid, said particulate material is circulated at aflow rate weight ratio to each of said streams in the range of fromabout 1 to about 20, and said second gas stream has a flow rate weightratio to said first gas stream in the range of from about 1.0 to about5.0.
 35. A method of heat transfer between separate streams of flowinggaseous media using a particulate material which is circulated betweenfirst and second operating chambers for respectively contacting firstand second gaseous streams therein, comprising:(a) flowing saidparticulate material and first gaseous stream into said first operatingchamber with contact between said particulate material and first gaseousstream to effect direct heat transfer therebetween, separating saidparticulate material from said first gaseous stream following contacttherewith and collecting said particulate material for transfer to saidsecond operating chamber; (b) flowing said particulate material andsecond gaseous stream into said second operating chamber with contactbetween said particulate material and second gaseous stream to effectdirect heat transfer therebetween, separating said particulate materialfrom said second gaseous stream and collecting said particulate materialfor return to said first operating chamber; (c) monitoring the pressuredrop of at least one of said gaseous streams and introducing saidparticulate material into said at least one gaseous stream as a functionof said pressure drop; and (d) cocurrently flowing said particulatematerial with each of said gaseous streams during contact therewith toeffect direct cocurrent heat transfer.
 36. A method as set forth inclaim 35, wherein each of said gaseous streams pneumatically conveyssaid particulate material during contact therewith.
 37. A method as setforth in claim 36, wherein said particulate material is circulatedsubstantially solely by each of said gaseous streams and the force ofgravity.
 38. A method as set forth in claim 35, including transferringsaid particulate material between said operating chambers throughsubstantially structurally unobstructed flow conduits and forming ineach of said conduits a column of particulate material having asufficient bulk density to prevent the net leakage flow of said firstand second gaseous streams between said operating chambers.
 39. A methodas set forth in claim 35, including violently mixing said particulatematerial and each of said gaseous streams respectively in at least onelocalized zone during cocurrent heat exchange therewith to maximize thequantity of heat transfer.
 40. A method as set forth in claim 35,wherein said pressure drop is inversely proportional to the weight flowrate of said at least one gaseous stream.
 41. A method as set forth inclaim 40, wherein the other of said gaseous streams comprises a hot fluegas having a temperature in excess of 750° F.
 42. A method as set forthin claim 40, wherein the other of said gaseous streams comprises hotprocess gas.
 43. A heat exchanger apparatus for providing heat transferbetween separate streams of gaseous media comprising first and secondvessels having a particulate material circulated therebetween in afigure 8 flow pattern, first and second lift lines for respectivelyconfining said streams as each of said streams pneumatically conveyssaid particulate material with cocurrent heat transfer to an associatedone of said vessels, downcomers respectively associated with each ofsaid vessels, each of said downcomers having an upper inlet forwithdrawing particulate material from said vessel and a lower outlet fortransferring particulate material to an associated one of said liftlines for pneumatic conveyance of said particulate material from one ofsaid vessels to the other of said vessels, each of sad downcomers havinga substantially unobstructed cross sectional area including asubstantially vertical portion between said upper inlet and lower outletfor confining said particulate material in a loose packed bed column anda laterally extending portion for accumulating and transferring saidparticulate material to said associated lift line, and separation meansassociated with each of said vessels for separating said streams andparticulate material and collecting said particulate material in saidvessels for withdrawal by said downcomers.
 44. A heat exchangerapparatus as set forth in claim 43, including means for violently mixingsaid particulate material and stream in at least one localized zoneduring pneumatic conveyance to each of said vessels to cause the heattransfer between the gaseous media and particulate material to besubstantially completed during pneumatic conveyance.
 45. A heatexchanger as set forth in claim 44, wherein said means for violentlymixing said particulate material and gaseous medium include a flowdiverting means disposed adjacent an outlet of each of said lift lines.46. A heat exchanger as set forth in claim 45, wherein said flowdiverting means comprise a bonnet member for entrapping gaseous mediaand particulate material.
 47. A heat exchanger as set forth in claim 44,wherein each of said downcomers includes biasing means for disturbingaccumulated particulate material in said laterally extending portionthereof and causing said particulate material to directly spill throughsaid outlet opening and into said stream of gaseous medium in saidassociated lift line.
 48. A heat exchanger as set forth in claim 44,wherein said separation means include a cyclone member having a dip legfor directly returning separated particulate material to said associateddowncomer.
 49. A heat exchanger as set forth in claim 44, wherein saidfirst and second vessels comprise a pair of horizontal cyclones.
 50. Amethod of heat exchange between separate streams of gaseous media usingparticulate material which is circulated along a continuous flow pathfor separate contact at spaced locations along the flow path with firstand second gaseous streams comprising, introducing said particulatematerial into said first gaseous stream at a first location in said flowpath for direct contact and heat transfer with the first gaseous streamas the particulate material is circulated along the flow path to asecond location therein, separating said particulate material from saidfirst gaseous stream at said second location and providing in said flowpath a loose packed bed column of said particulate which terminates at alower accumulation of particulate material in its natural angle ofrepose, introducing said particulate material into said second gaseousstream by biasing said particulate material from the accumulationthereof directly into said stream for cocurrent heat transfer with saidsecond gaseous stream as the particulate material is pneumaticallyconveyed along the flow path to said first location by said secondgaseous stream, and separating said particulate material from saidsecond gaseous stream at said first location and introducing theparticulate material into said first gaseous stream, the step of biasingsaid particulate material from said accumulation including monitoringthe pressure drop of said second gaseous stream during pneumaticconveyance and biasing said particulate material into said stream as afunction of the pressure drop to control the flow of particulatematerial along the flow path.
 51. A method as set forth in claim 50,wherein said first gaseous stream is a heat providing fluid comprising ahot flue gas, and the step of biasing said particulate material includescirculating said particulate material at a flow rate weight ratio tosaid first gaseous stream in the range of from about 3 to about
 15. 52.A method as set forth in claim 50, wherein the step of providing saidloose packed bed column includes adjustably confining said particulatematerial adjacent said accumulation thereof in said flow path to varythe natural angle of respose.
 53. A method of heat exchange betweenseparate streams of gaseous media in a system having heat exchangeoperating variables including gaseous media outlet temperatures andpressure drops, comprising circulating particulate material whichprovides a heat exchange surface along a continuous flow path forseparate contact at spaced locations along the flow path with first andsecond gaseous streams, introducing said particulate material into saidfirst gaseous stream at a first location in said flow path for directcontact and heat transfer with the first gaseous stream as theparticulate material is circulated along the flow path to a secondlocation therein, separating said particulate material from said firstgaseous stream at said second location and introducing said particulatematerial into said second gaseous stream for direct contact and heattransfer with said second gaseous stream as the particulate material iscirculated along the flow path to said first location, separating saidparticulate material from said second gaseous stream at said firstlocation for introduction of the particulate material into said firstgaseous stream, cocurrently exchanging heat between said particulatematerial and at least one of said gaseous streams during contacttherewith, sensing at least one of said heat exchange operatingvariables, and varying the area of said heat exchange surface inaccordance with the sensed variable's deviation from a preselected valueby regulating the introduction of said particulate material into saidgaseous streams at said first and second locations.
 54. A method as setforth in claim 53, wherein the step of varying the area of said heatexchange surface includes regulating the introduction of saidparticulate material into said first gaseous stream in accordance withthe sensed variable's deviation from said preselected value andregulating the introduction of said particulate material into saidsecond gaseous stream to correspond with the introduction of particulatematerial into said first gaseous stream.
 55. A method as set forth inclaim 53, wherein said method of heat exchange is employed in a morecomprehensive system having process operating variables includinggaseous stream outlet temperatures and gaseous stream pressure drops,and said sensed heat exchange operating variable is a combined heatexchange and process operating variable.
 56. A method as set forth inclaim 55, wherein said combined heat exchange and process operatingvariable is a total system pressure drop.
 57. A method of heat transferbetween separate streams of flowing gaseous media in a system havingheat exchange operating variables including gaseous media outlettemperatures and pressure drops, wherein a particulate material providesa heat exchange surface as it is circulated between first and secondoperating chambers for respectively contacting first and second gaseousstreams therein, comprising:(a) causing said particulate material andfirst gaseous stream to flow into said first operating chamber withcontact between said particulate material and first gaseous stream toeffect direct heat transfer therebetween, separating said particulatematerial from said first gaseous stream following contact therewith andcollecting said particulate material for transfer to said secondoperating chamber; (b) causing said particulate material and secondgaseous stream to flow into said second operating chamber with contactbetween said particulate material and second gaseous stream to effectdirect heat transfer therebetween, separating said particulate materialfrom said second gaseous stream and collecting said particulate materialfor return to said first operating chamber; (c) effecting cocurrent flowof said particulate material with at least one of said gaseous streamsduring contact therewith to effect direct cocurrent heat transfer; (d)sensing at least one of said heat exchange operating variables; and (e)varying the area of said heat exchange surface in accordance with thesensed variable's deviation from a preselected value by controlling theflow of said particulate material into each of said operating chambers.58. A method as set forth in claim 53 or claim 57, including cocurrentlyexchanging heat between said particulate material and the other of saidgaseous streams.